continuity hypothesis definition psychology

CONTINUITY HYPOTHESIS

1. the presumption that effective discrimination learning or problem resolution stems from an advanced, step-by-step, ongoing process of experimentation . Reactions which turn out to be unsuccessful are ceased. On the other hand, any strengthened reaction results in an advancement in associative endurance, thereby yielding the slow but ongoing elevation of the learning curve . Problem resolution is developed as a gradual learning procedure wherein the accurate answer is found, repeated, and strengthened. 2. the assertion that psychological procedures of numerous types occur either in small measures or continually, instead of in spurts from one recognizable phase to another.

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The Continuity Hypothesis of Dreams: A More Balanced Account

Dream elements are both continuous and discontinuous with waking life..

Posted September 10, 2014 | Reviewed by Jessica Schrader

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The continuity hypothesis of dreams suggests that the content of dreams are largely continuous with waking concepts and concerns of the dreamer. In my previous posts on the continuity hypothesis, I have been a bit unfair presenting only a case against the hypothesis and not laying out the facts and arguments that support the hypothesis. So I want to begin to redress that imbalance the best I can in the present post.

Calvin Hall was the first dream researcher to argue that some contents of dreams reflected the daily concerns and ideas of the dreamers rather than the hidden libidinal wishes or compensatory emotional strategies that psychodynamic theorists like Freud and Jung advocated. Through creation of standardized dream content scoring inventories (building on the work of Mary Calkins and others), Hall demonstrated that the most frequently appearing content items of dreams were not bizarre images at all but rather mundane social interactions between the dreamer and people he or she interacted with on a daily basis. One did not need to invoke theories concerning elaborate dreamwork to disguise latent libidinal and aggressive wishes buried in the dream.

Instead simple counts of characters, interactions, objects, actions and events in the dreams could yield a pretty accurate picture of what the dream was about and it wasn’t dramatically different than the daily life of the dreamer. Many dream researchers since Calvin have confirmed that the bread and butter of dreams are the quotidian daily social interactions and concerns most people experience on a daily basis. Domhoff’s (2003) impressive content analyses of a longitudinal dream series collected from a middle aged woman dubbed “Barb Sanders” very convincingly shows that her pattern of aggressive and friendly interactions with key characters in her dreams matched the ups and downs of those same relationships between her and them in waking life.

Thus the empirical support for some degree of continuity between dream content and waking life is strong. The database supporting the theory has been considerably strengthened by many dream researchers over the years since Hall’s pioneering efforts back in the 1950s-1970s. It is therefore clear that any complete theory of dreams must accommodate the data demonstrating substantial continuities between dream content and waking concepts and concerns.

But as every supporter of continuity theory acknowledges there are also dreams that contain some significant discontinuities between dream content and waking concepts/concerns. For example, most people have had dreams that are like long adventure stories or movies. These “narrative-driven” dreams are less quotidian than everyday dreams. They contain more bizarre elements and imagery and have the dreamer engaged in actions and events that are decidedly not like their ordinary ideas, actions and concerns. In addition, there is a significant minority of dream reports that have few or no familiar characters, settings, or activities. Can these sorts of dreams be explained with continuity theory approaches? If attempts are made to do so how can one avoid special pleading, circular reasoning or ad hoc additions to the theory?

What's needed is a theory that accommodates both continuities and discontinuities, but I don't see one of the horizon. In the meantime, one proponent of the continuity hypothesis suggests that building toward such a theory should start with the assumption of continuity, followed by a search for discontinuities that might be of varying types, such as narrative/adventure dreams, unusual elements that reveal figurative thought, and incongruous elements that may reveal that there are cognitive defects of various sorts in dreams (Domhoff, 2007).

Domhoff, G. W. (2003). The scientific study of dreams: Neural networks, cognitive development, and content analysis. Washington, DC: American Psychological Association.

Domhoff, G. W. (2007). Realistic simulation and bizarreness in dream content: Past findings and suggestions for future research. In D. Barrett & P. McNamara (Eds.), The new science of dreaming: Content, recall, and personality cor¬relates (Vol. 2, pp. 1-27). Westport, CT: Praeger.

Patrick McNamara, Ph.D. , is Associate Professor of Neurology at Boston University School of Medicine and the author of numerous books and articles on the science of dreams.

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Continuity in Psychology (Definition + Examples)

When we consider continuity within the realm of psychology, we regard it concerning the principles of Gestalt. As with all psychological theories, that of continuity is accepted by some and rejected by others. Let us explore the meaning of continuity psychology and its implications for our consciousness and daily lives.

Continuity psychology, as proposed by Gestalt, refers to the theory that our brains detect what we experience or see as continuous even if it is, in reality, disjointed. This theory can be applied to our character and personality, implying that what we experience growing up defines our adult identity.

Continuity means that something continues indefinitely. It can also describe how our eyes follow a line until it is interrupted. When we apply the theory of continuity, it adds a different flavor to what we think about. For some, the idea implies that our identity is linked to the continuity of our memories. For others, the matter is a little more complicated. Hold onto your hat as we dive into the meaning of continuity psychology, Gestalt's theories, and what all this means for our lives.

Continuity Psychology Meaning

Simply put, the principle of continuity is our brain's desire and ability to create complete, continuous things from disjointed ones.

Considering how a movie or music is made, we can see these things are made of many small, disjointed parts. A movie is created using millions of still images shown in succession. Music is made of individual notes played by different instruments, one after the other.

Still, our brains discern an entire movie or piece of music as one complete product. Our brains have created continuity from parts of a whole.

Our brains attempt to understand an image as quickly as possible when we see it. This action sometimes leads us to see things we expect to be there instead of those that indeed are. For example, we may be shown a complex pattern and discern known shapes within it, like triangles or faces. In reality, it may simply be a pattern with no distinct images.

Our brains work this way to help us access information and gain understanding as quickly and efficiently as possible.

The principle of continuity is one of five theorized by a psychologist named Gestalt. All five principles show how our brains create shortcuts to help us understand the world around us.

The five Gestalt principles are:

• Connectedness

Each of these five principles works similarly to that of continuity. Let us look briefly at all five to understand our primary focus, continuity, better.

1.      Proximity

In the principle of proximity, we see that if items are grouped, our brains determine that they belong together. In this case, all the things that appear in groups may be identical. Simply the proximity to each other determines their status of belonging.

If items are identical but further away than our brains determine it is eligible for belonging, the items will be deemed entirely separate.

2.      Similarity

The principle of similarity refers to how we group items. This process begins early on as we start to group toys according to soft, plastic, color, those that belong in the bath, etc. We may group items according to any criteria that make sense and tend to group according to similarities.

3.      Continuity

The third principle is continuity, and we have already touched on this. We will go into much more detail throughout this article; however, it refers to our brain's capacity to create a continuous end-product from pieces provided.

4.      Connectedness

In this principle, we understand that we can make connections from fragments of a complete image. When our forefathers could create images from the constellations, they employed connectedness.

When we can make or witness a collage or mosaic, our brains are helping us to see the complete image made from smaller parts. In this case, we are using the principle of connectedness.

5.      Closure

We often see puzzles on social media asking us what is missing or wrong with a specific image. 

In most cases, the creators of the puzzle rely on our strong sense of closure to mess with our ability to solve their challenges.

The principle of closure is when our brains fill in the blanks to create an end-product we deem satisfactory. One example would be seeing the alphabet with one missing letter. We may struggle to see which letter is missing since our brains fill in the blank.

When editors work on written work, they often need to read out loud because of this phenomenon. 

If the editor reads silently, their brain automatically fills in the blanks and self-corrects the errors in the piece. When the editor reads aloud, they bypass the brain's desire for closure and see the errors that are blatantly evident in the text.

Now that we have briefly learned about all five of Gestalt's principles let us look into the continuity principle and its application in psychology.

Continuity Principle In Psychology

The principle of continuity is intriguing when we look at how our brains assist us in our daily lives. 

What may otherwise seem disjointed and unappealing is made entertaining and enjoyable to behold, thanks to our desire for continuity.

When considering its application in psychology, we are led to wonder how far we can take our brain's desire for continuity and how much weight it holds for the understanding we possess of ourselves and our world.

Furthermore, theorists have claimed that continuity, in terms of our memories, is what makes up our character and identity. Let us look into all of these elements to gain a clear understanding of the theories at play.

Continuity Of Cognition Throughout Our Lives

Have you ever noticed that, as much as you may change over the years, you continue to be drawn to similar people, styles, activities, subjects, and more? This could be because your ideas of what you enjoy and value were set when you were young.

A theory claims that our experiences create who we are and remain with us throughout our lifetimes. One reason for this is our desire for continuity.

Since our brains desire continuous flow, the desire may be strong enough to carry into long-lasting effects such as preferences. Could it be that our taste in music, for example, has indeed changed, but our brains are keeping us in a loop of enjoying the same genre for the sake of continuity?

If we understand this principle, we should realize Locke's theory. He proposed that our entire identity is tied up with psychological continuity and that our memory is, in fact, our consciousness.

Locke's theory is similar to Gestalt's since both propose that our current identity and understanding of the world are based on continuity and what we have experienced.

According to Maria Montessori, an education specialist, children are in the period of the absorbent mind until around six years old. During this time, they learn most of what they will need to carry them through life. It was Piaget who agreed that children should first be taught the basics and then be allowed to experiment with them.

Perhaps these education specialists understood the principle of continuity and how it affects us psychologically. Maybe it was their personal desire for continuity, or the observance of it in children, that led them to their educational theories that tie in with the principle.

Continuity In Psychological Treatment

The idea of continuity psychology is evident in counseling and therapy practices globally. Patients often begin counseling or therapy out of a desperate need for clarity or help in their current situation. What typically follows is a regression to the patient's past.

The counselor or psychologist can piece together parts of their life by looking at the patient's childhood activities, traumas, treatment, joys, troubles, and more. This keyhole glance at the inner workings of the patient allows the counselor or therapist to ascertain what parts of the patient have carried over into their current life.

In many cases, a counselor or psychologist can explain to the patient what they have carried into their current life and show them how their desire for continuity has led them to their present situation.

If the patient can grasp the theory well, they can select the good from the bad of their past and move forward intentionally.

Plato explained our consciousness as like being in a cave. In this cave, shadows move, and events occur, but our understanding is limited to what we have seen before and therefore understand.

Living in an unconscious state of continuity is similar. We think and act based on what we have experienced before but do not think consciously about our current choices and ways of thinking and existing.

Continuity is sometimes a life-saving safety net. It allows us to function and perform high-order tasks without reinventing our mindset with each new challenge. It can, however, also lead to an element of laziness and a degree of blindness if we do not participate in our consciousness.

Continuity As The Basis For Consciousness And Identity

Gestalt and Locke describe continuity as the basis for our identity. They theorize that our memories of past events make up our current character and personality.

The definition of continuity psychology is the ability to continue with something in the same way, indefinitely. If this is the case with our identity, it could lend weight to the theory of life after death or reincarnation. The word "indefinite" implies that even the end of a physical body could not stop a consciousness.  

Reid disagreed with Gestalt and Locke, stating that, since he cannot remember every detail of his life, his memory cannot possibly make up the entirety of his consciousness. He questioned whether forgetting something meant it had not happened at all.

Furthermore, he asked if he had forgotten something, but someone else remembered it about him, was that memory then a part of the other person's consciousness and no longer his.

As we can see, the idea that memory and continuity are the basis for consciousness is rather complicated and not to be taken too literally.

While our memories add to our identity, they cannot be the sole creators of it. Our brains use our memories and help us piece together our identities based on what we understand throughout our lives.  

An example would be as follows:

A child grows up in a family with parents who were hippies in the 1970s. She hears the era's music playing and conversations about rebellion and notices elements of her parents' personalities that tie in with the hippie lifestyle of the time.

As she grows up, her tastes lean toward hippy, bohemian trends, and she enjoys similar music, fashion, art, and career choices. Her identity is wrapped up in her childhood experiences with her parents.

Over the years, she learns more about the era and her parents as people and realizes there is likely a lot more to who her parents were and what they did.  

She is now faced with a choice. The desire for continuity will help her filter out the "bad" elements she has learned and focus only on the good. In this case, she will continue as before, living in denial.

She may choose to dig deeper and discover more of the truths she has begun to uncover. Once she has learned a great deal, she can then decide whether or not her previous identity still fits or if she would like to develop a slightly different one.

It is vital to understand that, hard as we may work to change, our development away from any initial identity or sense of self is less of a significant shift and more like a gradual evolution.

While we may make conscious changes to our lifestyle and understanding of things, the forces of continuity, culture, identity, and memory are strong and deeply engrained. In the end, while we may not like all that we discover about our histories and from where we acquired our consciousness, we should try to appreciate every element of ourselves.

Continuity psychology rests on the understanding that our early experiences form our consciousness. Our brain's overwhelming desire for continuity and sameness in an ongoing fashion leads us to continue on paths we began many years ago unconsciously.

Continuity is one of five principles theorized by Gestalt. All the principles he proposes are ways for our brains to make shortcuts to understanding our experiences and the world around us. If we are to live in a genuinely conscious state, we should frequently question our decisions and automatic thoughts.

https://study.com/academy/answer/the-principles-of-continuity-and-closure-best-illustrate-what.html

https://www.betterhelp.com/advice/psychologists/what-is-continuity-psychology/

https://www.masterclass.com/articles/allegory-of-the-cave-explained#quiz-0

https://www.merriam-webster.com/dictionary/continuity

https://www.usertesting.com/resources/topics/gestalt-principles#:~:text=The%20principle%20of%20continuity%20states,on%20the%20line%20or%20curve.

https://outsidetheboxmom.com/what-is-continuity-psychology/

https://www.thoughtco.com/montessori-method-4774801

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This article explains the origins and development of the continuity hypothesis in work by cognitively oriented dream researchers. Using blind quantitative analyses of lengthy dream series from several individuals, in conjunction with inferences presented to the individual dreamers to corroborate or reject, these researchers discovered that the same conceptions and personal concerns that animate waking thought are very often enacted in dreams. Other types of studies later supported this finding. The article argues that the cognitive origins and definition of the continuity hypothesis have been distorted by those dream researchers who mistakenly claim that the concept is focused on dreaming as an incorporation of everyday experiences. A review of the literature on experiential and experimental influences on dreams, which includes studies of day residues, the experimental manipulation of presleep events, the incorporation of during-sleep stimuli, laboratory references in laboratory-collected dreams, and the influence of routine daily events, reveals that none of them is very influential and most are trivial. The article concludes that those who study experiential factors should adopt a phrase such as "incorporation hypothesis" to avoid confusion in the literature and make clear that the continuity hypothesis is a central one in an emerging neurocognitive theory of dreams. The intensity of personal concerns and interests, not the events of the day, shape central aspects of dream content. In particular, the frequency of characters or activities reveals the intensity of various concerns, and these concerns can be discovered for individuals through comparisons with normative findings.

The continuity hypothesis put forth in the early 1970s by pioneer dream researcher and cognitive theorist Calvin S. Hall and his coauthor (Bell & Hall, 1971) is widely known and discussed by dream researchers. In its fully developed form, as one key concept in a cognitive theory that has become a neurocognitive theory, the continuity hypothesis states that a large majority of dreams (the exact percentage has not yet been established) express the same conceptions (e.g., of oneself, of specific relatives and friends) and personal concerns (e.g., relationships, avocations) that animate waking thought (Domhoff, 1996, pp. 5, 153; 2001). Building on the working hypothesis that the frequency with which a character, type of social interaction, or activity appears in a series of dreams reveals the intensity of a concern, predictions can be made about the conceptions and personal concerns that are most important for both groups and individuals (Hall & Van de Castle, 1966, pp. 13-14).

More specifically, based on blind quantitative content analyses of a set or series of dream reports, this approach leads to the hypothesis that statistically significant differences between two sets of dream reports, or between an individual dreams series and normative baselines, will relate to the psychologically unique aspects of the dreamers' waking thoughts. Thus, the theory, based on a wealth of past findings, presents a clear statement of what is the most important influence on the frequency of specific types of content in dreams: the intensity of personal concerns.

However, despite the clear meaning that had been established for this concept within the context of a cognitive theory of dreaming by the late 1990s at the latest, it has been repeatedly misstated since that time by more recent dream researchers. The original misguided restatement mistakenly claimed that the essence of the concept is that dreams "reflect" waking-life experiences, with an emphasis on the "incorporation rate" of various types of experiences (Schredl, 2003, p. 26). This formulation is the opposite of dreams as being to a large extent the expression ( enactment and dramatization are often used as synonyms) of conceptions and concerns, some of which go back years and decades for many individual dreamers. Despite strong criticisms of this experientialist distortion of the cognitively oriented continuity hypothesis (Bulkeley, 2011, 2012; Domhoff, 2003, 2010b; Domhoff, Meyer-Gomes, & Schredl, 2006), this altered version has been uncritically adopted by many dream researchers without any mention of the original meaning of the concept or of the criticisms of the experiential incorporation version (e.g., King & DeCicco, 2009; Malinowski & Horton, 2011, 2014; Sándor, Szakadát, & Bódizs, 2016; Selterman, Apetroaia, Riela, & Aron, 2014). The problem also has been compounded in a follow-up statement defending and extending the original mistaken article (Schredl, 2012).

The mistaken redefinition of the continuity hypothesis has been translated into a strong indictment by dream researchers who see the continuity hypothesis as a "competitor" to their "threat simulation" and "social simulation" theories, both of which claim that dreams are rehearsals that are useful for dealing with future waking events (see Revonsuo, 2000; Revonsuo, Tuominen, & Valli, 2015, pp. 1, 11, 19, 24, for repeated statements that the continuity hypothesis is in competition with their views). Focusing primarily on the experiential distortion of the continuity hypothesis, they claim that the concept is "too vague and general as a theoretical explanation for the details of dream content," does not "predict in any detail how and why the causal relationship between waking and dreaming works," does not "specify in any detail what counts as a 'continuity' and what would count as a 'discontinuity,'" and "takes almost any similarity between waking life and dream life as a confirmation of the continuity hypothesis" (Revonsuo et al., 2015, p. 10).

In fact, these claims are false. The continuity hypothesis was not presented as a "theoretical explanation for the details of dream content" but as an empirical discovery that has been replicated numerous times, a discovery that can be explained by the idea that most dreams enact and dramatize the same conceptions, personal concerns, and personal interests that animate waking thought. (The word personal is usually connected with the mention of a "concern" or "interest" in this article because the theorists criticized in his article have stretched the original meaning of the term concern to include everyday concerns and passing daily interests.) The original version of the cognitive theory of dreams stated that dreams are the "embodiment of thoughts," and the more recent neurocognitive version states that dreams are embodied simulations that very frequently enact conceptions and personal concerns (Domhoff, 2011b, 2015, in press, Chapters 2-3; Hall, 1953, pp. 273-274). More specifically, perhaps as many as 70% of dreams may be embodied simulations of conceptions and personal concerns (Domhoff, in press, Chapter 2; Domhoff et al., 2006, Table 3, p. 276).

Because of the various misunderstandings and critiques of the continuity hypothesis, it is the purpose of this article to fully explain the origins and evolution of the continuity hypothesis as a key concept in a cognitive, and subsequently neurocognitive, theory of dreams (Domhoff, 1996, pp. 209-212; 2001, 2003, 2011b, in press). The article shows that the concept expresses a major disagreement with the predominant theorists of the clinical era of dream research, who stressed the discontinuities between dreaming and waking thought (Freud, 1900; Fromm, 1951; Jung, 1974). It explains that the concept had its origins in studies of several individual dream series, not in studies based on 2-week dream diaries and questionnaires, which are frequently used by its critics.

Although experimentally oriented skeptics sometimes claim that lengthy dream journals are selective samples that may reflect only the most memorable dreams the person recalls, as in the case of one anonymous reviewer of this article, they overlook the fact that several of these series contain three or more entries per week, which is in the range of the typical rate of dream recall (e.g., Beaulieu-Prévost & Zadra, 2005b; Beaulieu-Prévost & Zadra, 2015; Blagrove & Akehurst, 2000; Schredl, 2008). One of these series, with 2,022 dream reports over a 3-year period, often includes two or more dreams a night, for an average of 13 dream reports per week, far beyond the usual rate in most questionnaire and 2-week diary studies This series, which can be found on DreamBank.net under the pseudonym "Kenneth," leads to the same results that have been found in other studies (Domhoff, in press, Chapter 3; Domhoff & Schneider, 2008, p. 1243).

As noted at the outset, the use of blind quantitative analyses of lengthy dream series makes it possible to develop specific inferences about the dreamers' waking conceptions and personal concerns. These inferences then can be verified or rejected with extant biographical information or direct responses from the dreamers themselves. Because of mistaken inferences in some of the earliest studies making use of the concept, which are examined in the next section, the continuity hypothesis was further refined. It was then supported in later studies of longer dream series that included more detailed analyses and obtained more detailed responses to the inferences from the dreamers, along with testimony from friends of the dreamer in one instance (Bulkeley, 2012, 2014; Bulkeley & Domhoff, 2010; Domhoff, 2003, Chapter 5; 2015; in press, Chapters 3-4).

This defense of the established cognitive meaning of the continuity hypothesis is necessary because the broader definition of the concept downplays its cognitive meaning, opens the door to very loose uses of the term "continuity" by new dream researchers, and creates confusion for all dream researchers. The concluding section of the article suggests that those who include daily events, daily concerns, or episodic daily interests within the purview of their interest in continuity should use a term such as experiential orientation hypothesis , incorporation hypothesis , experiential incorporation hypothesis , or some equivalent term. They should do so because the continuity hypothesis has become an important ingredient in a cognitive tradition that envisions dreaming, along with mind-wandering and daydreaming, as a form of spontaneous, stimulus-independent thought, a tradition that accords little if any weight to everyday daytime experiences (e.g., Antrobus, Singer, Goldstein, & Fortgang, 1970; Domhoff, in press; Foulkes, 1985; Fox, Nijeboer, Solomonova, Domhoff, & Christoff, 2013).

The Origins and Later Refinement of the Continuity Hypothesis

To appreciate the importance of the continuity hypothesis, it is necessary to explain the empirical and theoretical context within which it arose as well as to report the refinements that were made to it, which are seldom or never discussed by those who have altered the meaning of the concept. Such a discussion makes it possible to show how the continuity hypothesis played a key role in the expansion of the original version of Hall's (1953) cognitive theory of dreams into the present-day idea that dreams are embodied simulations that dramatize conceptions and personal concerns (Domhoff, 1996, pp. 209-212; 2007, 2015). In the first statement of his cognitive theory of dreams, as noted in the introduction, Hall (1953, pp. 273-274) claimed that dreams are "the embodiment of thoughts" and then added that thoughts are based on "conceptions," which he explained as follows: "Thinking is a process of conceiving. The end-product of this process is a conception (idea). A conception is an item of knowledge, a formulation of experience that has meaning for a person" (Hall, 1953, p. 274).

Although the cognitive theory of dreams was first stated in 1953, the continuity hypothesis was only developed many years later in a context in which studies attempting to test psychodynamic ideas about dreams through the use of projective or paper-and-pencil measures of personality had led to meager findings. Most of these disappointing results, many of which were reported by Hall's dissertation students at Case Western University in the late 1940s and early 1950s, are summarized in a chapter in a volume on Progress in Clinical Psychology (Hall, 1956). The list of failed or inconclusive studies was updated 40 years later, although there were few or no studies undertaken after 1980 (Domhoff, 1996, pp. 154-156).

In the mid-1960s, in a new introduction to the second edition of his popular book, The Meaning of Dreams , Hall (1953/1966) further reported that he had been very wrong in his inferences in a recent study of the personalities and leadership qualities of 2 of 17 mountain climbers who wrote down their dreams for several weeks as one part of a larger study of the 1963 American Mount Everest Expedition. Although his assessments of most of the men were consistent with those of the psychologists who studied their waking behavior, the two men he thought were the most popular, the most psychologically mature, and the most effective leaders turned out to be "the least liked, the most immature, and had no leadership or morale building assets whatsoever" (Hall, 1953/1966, p. xx). He called it a "sobering experience" to discover "the enormity of the misjudgments that can be made in assessing a person's waking behavior [italics added] from his dreams" (Hall, 1953/1966, p. xx).

Thus, despite Hall's (1954; Hall & Nordby, 1973) admiration for both Freud (1900) and Jung (1963, 1974), and his use of some of their ideas, the meager or negative findings with the standard personality assessments used in psychological research, along with his own mistakes, caused him to amend his blend of cognitive and psychodynamic theory in an even more cognitive direction. However, it was not until he began to study dream series from adults about whom he could obtain autobiographical or biographical information, or who were available to respond to the inferences about current conceptions and personal concerns (based on blind analyses of their dream series), that he made any progress in understanding the specifics of the relationship between dreaming and waking thought and to conceptualize this relationship in terms of "continuity." Although dream series were first used by the early Freudian Wilhelm Stekel (Hall, 1953/1966, p. 236) and Carl Jung (1963, 1968, 1974), their use was novel in terms of empirical dream research based on quantitative content analysis.

The first published scholarly statement related to Hall's gradual development of the continuity hypothesis appeared in a book coauthored with Richard Lind on the Dreams, Life, and Literature of Franz Kafka, which was based on a content analysis of the 37 dreams found in the Kafka diaries that had been published as of the 1960s (Hall & Lind, 1970). After making various predictions about Kakfa's waking conceptions and personal concerns, which derived from a comparison of the content findings for Kafka with the male norms in Hall and Van de Castle's (1966) comprehensive quantitative coding system, Hall and Lind (1970) then turned to a careful reading of Kafka's diaries and letters, which they had purposely ignored up to this point, as well as to a reading of biographies and remembrances of Kafka.

The final chapter, entitled "Realizations," begins by noting that "modern dream theories," for all their differences, emphasize that dreams are "discontinuous with waking life" (Hall & Lind, 1970, p. 89). In other words, Hall thought that Freud, Jung, and most of the neo-Freudians overemphasized the discontinuities between waking thought and dreams in their theorizing. Freud (1900, 1901) did so through the idea that adult dreams are heavily disguised and opaque to the waking mind because they express repressed sexual and aggressive desires that have to be disguised by "the dream-work." This reworking of the repressed wishes by the four cognitive processes that comprise the dream-work makes it necessary for a Freudian-trained expert to find the meaning of any given dream, usually through obtaining free associations to each element of the dream and being alert for what are now called figurative meanings (Gibbs, 1994; Lakoff, 1987).

As for Jung (e.g., 1963, 1974; Mattoon, 1978), he claimed that important dreams were if anything the opposite of waking thoughts because they compensated for the underdeveloped aspects of the person's psyche in very arcane ways (as revealed via symbolism). This theory made expert knowledge about mythology, religious history, and general metaphoric meanings necessary for developing an understanding of dreams. Finally, neo-Freudians, through their emphasis on the alleged metaphoric nature of dreaming, also found it necessary to draw upon myths, fairy tales, and metaphors to understand the underlying meaning of dreams (Fromm, 1951). (There were a few minor dissident neo-Freudians who dissented from the emphasis on discontinuity, but their claims for continuity were based on clinical case studies and had no lasting impact [e.g., French & Fromm, 1964]). Contrary to the well-known clinical theorists who emphasized discontinuity, who far and away predominated for the first 6 decades of the 20th century, Hall and Lind (1970) put their emphasis on the following statement: "This study of Kafka's dreams in relation to his life indicates that dreams are more likely to be continuous with waking life" (Hall & Lind, 1970, p. 89). This conclusion is a frank disagreement with the reigning discontinuity theorists. This point explains Hall's emphasis on "continuity" and puts his choice of terms in the historical context of the past discontinuity theorists. The statement also provides the explanation for a brief, seemingly cryptic sentence that appears in the discussion of the continuity hypothesis in a coauthored popular book with his research assistant: "The dream world is neither discontinuous [i.e., Freudian] nor inverse [i.e., Jungian] in its relationship to the conscious world" (Hall & Nordby, 1972, p. 104). However, the concept of a "continuity hypothesis" was not introduced in the book on Kafka's dreams.

The Kafka book was followed a year later by Hall's study of 1,368 dreams that a child molester wrote down over a 4-year period between 1963 and 1967 for his own reasons; only later did he give them to a clinical psychologist, Alan Bell, who interviewed him at the prison mental hospital in which he was incarcerated (Bell & Hall, 1971). Because of the huge sample size, Hall was able to make many inferences that were for the most part supported by personality assessment instruments and clinical materials collected by Bell at the facility as well as through the dreamer's written replies to questions formulated by Bell and Hall (1971, Chapter 2).

In this study Hall drew on the earlier discovery that the frequencies of an element in a series or set of dream reports reveal the relative intensity of waking concerns (Hall & Van de Castle, 1966, pp. 13-14). This finding adds a quantitative precision to the concept of "concerns," which should not go unremarked here because it makes it possible to rank personal concerns and demonstrate in yet another way that dreams are not based on incorporations from everyday waking life. For example, if a man dreams far more frequently about his mother than is the case for the male norms, or than any other character in a dream series, as this dreamer did, "it is inferred that the mother plays an important role in his life," as was indeed the case in this instance (Bell & Hall, 1971, p. 117). There then follows the first mention of the continuity hypothesis, with the italics in the original: "This may be called the continuity hypothesis because it assumes there is a continuity between dreams and waking life" (Bell & Hall, 1971, p. 117).

It is also important to add that the quoted sentence is followed by a qualifying statement showing that further work was needed to refine the concept: "There are difficulties with this hypothesis as we have seen, but first let us reconsider some of the kinds of information that dreams provide" (Bell & Hall, 1971, p. 117). The "difficulties" that Hall alludes to concern the fact that the continuity is not always with both waking thought and behavior, but sometimes with waking thought only, which is one of the key reasons why the emphasis came to be on waking thought, not behavior, in later refinements of the concept (Domhoff, 1996, 2003). That is, a behavioral concomitant may or may not be present, but the same concern expressed in the dream reports is always present in waking thought.

The child molester study, although a major advance over the Kafka study, nonetheless had deficiencies, partly because Bell, reasonably enough, wanted to foreground the projective tests, paper-and-pencil personality tests, interviews, and clinical information he had collected to construct a portrait of the child molester's personality (see Bell & Hall, 1971, p. 8 for the different interests and the division of labor between the two authors). More importantly, however, the analysis was presented within a Freudian framework, which led among other things to Hall's inference that the complete absence of the dreamer's father from the dream reports must mean that his conceptions and concerns in relation to his father had been repressed (Bell & Hall, 1971, p. 73). He then concluded that this claim was supported by the child molester's belief that he had been sexually abused by his father at age 4 (Bell & Hall, 1971, pp. 24, 76-77, 90).

However, this is the kind of memory of the past that is difficult if not impossible to confirm, as shown by the numerous later studies of the fallibility of memory (e.g., see Bauer, 2013; Garry & Hayne, 2007, for overviews). This is perhaps even more the case with highly troubling "recovered" memories from childhood, most if not all of which prove to be inaccurate, and especially so if they are allegedly from before the age of 5 or 6 (e.g., Foley, 2013; Howe, 2000; Howe & Knott, 2015). They are constructed in various ways through the imaginative reworking of memories and suggestions, which are often due to pressures from legal investigators of alleged molestations or through interactions with therapists who believe that child molestation is widely prevalent and is perpetrated even by women working in nursery schools (e.g., Ceci, Bruck, & Battin, 2000; Hyman & Loftus, 2002; Loftus, Joslyn, & Polage, 1998; Loftus & Ketcham, 1994; Ofshe & Watters, 1994; Ornstein, Ceci, & Loftus, 1998; Strange, Clifasefi, & Garry, 2007).

Thus, later cognitive dream researchers minimized or entirely abandoned any inferences and questions about long-ago events that allegedly happened to the dreamer. In terms of the inferences that they do make, the focus is on ongoing personal concerns. At the same time, it was never doubted by cognitive theorists that many of these personal concerns were persistent ones that the dreamers believed to go back many decades, especially in terms of past relationships with parents and siblings (Domhoff, 1996, Chapters 7-8; 2003, Chapter 5). Therefore, they rejected any claim that past concerns had been repressed, only to be recalled much later with the help of hypnosis or psychotherapy and then labeled a recovered memory. By the same token, the zero-frequency hypothesis put forth in the case of the child molester, based on the concept of repression, for which there is no systematic empirical evidence, was also discarded by later cognitive dream researchers (Loftus et al., 1998; Loftus & Ketcham, 1994; Mazzoni & Loftus, 1998; Mazzoni, Loftus, Seitz, & Lynn, 1999; Ofshe & Watters, 1994).

The first complete statement of the continuity hypothesis was put forth in a popular book in 1972: " The wishes and fears that determine our actions and thoughts in everyday life also determine what we will dream about [italics added]" (Hall & Nordby, 1972, p. 104). Hall then did several other studies related to the continuity hypothesis, adding new wrinkles and evidence, until shortly before his death in 1985; most of these studies were not published. On the basis of these studies, his final published statement on the continuity hypotheses used the phrase "concerns and preoccupations" instead of "wishes and fears," which reflected his gradual distancing from the language of the two major psychodynamic theorists, Freud (1900) and Jung (1974): "The continuity hypothesis states that dreaming is continuous with waking life. That is, people will manifest in their dreams the concerns and preoccupations of their waking life [italics added]" (Hall, Domhoff, Blick, & Weesner, 1982, p. 193).

Hall's further development of the continuity hypothesis, and the changes in it by cognitive theorists, is traced out in greater detail in a chapter in Finding Meaning in Dreams entitled "The Continuity Between Dreams and Waking Life" (Domhoff, 1996, Chapter 8). In particular, this chapter spells out the mistaken inferences Hall drew and the lessons that he and later cognitive dream researchers learned from them. They are important in understanding how the concept evolved into a fully cognitive one, with no remnants of the psychodynamic elements in Hall's earlier theorizing.

For example, in the case of the child molester, Hall inferred (based on several dreams in which the dreamer was masturbating) that he was a frequent and compulsive masturbator. However, the dreamer said this was untrue and noted that he was able to resist urges to masturbate for weeks at a time. He thought masturbation was wrong and often felt depressed after he masturbated (Bell & Hall, 1971, pp. 25, 94). He worked hard to overcome his preoccupation with his body and was very interested in spirituality and meditation. Bell and Hall (1971, p. 96) therefore concluded that "an analysis of the dream content reveals very little about his defensive maneuvers," which means that he was able to resist masturbating in waking life. This distinction is a very useful one that was taken into account in future studies by those who followed in Hall's path. It is one of many pieces of evidence that refutes the claim that the continuity hypothesis has any necessary relationship to waking behavior. This evidence includes a more recent study also concluding that the frequency of sexual dreams was related to the frequency of waking sexual fantasies, but not to the frequency of masturbation or sexual relations (Schredl, Desch, Röming, & Spachmann, 2009).

Hall also did a study of 58 dreams from a neurotic patient in psychotherapy in which his 42 inferences were corroborated or rejected by the psychotherapist (Domhoff, 1996, pp. 171-176; Moss, 1970, Chapter 3). Most of his inferences proved to be correct, but it is his mistakes that were the most useful. In particular, he wrongly inferred on the basis of only one dream, a dream in which the patient and his brother were trapped in their mother's apartment (i.e., she was not actually present in the dream), that the dreamer had a negative relationship with his mother. This inference proved to be quite wrong. In effect, Hall overgeneralized, as he explained: "I figured that anyone who had such poor relations with women, including his wife, and then has a dream in which he is trapped in her [i.e., his mother's] apartment, must also have poor relations with his mother" (Domhoff, 1996, p. 175). The lesson here is that hostility toward a general category, such as women, should not be presumed to include specific significant others who are included in that category unless there is hostility toward them as well. In effect, Hall relied on psychodynamic theory in making this inference, as can be seen in his sentence quoted earlier in this paragraph, rather than relying on specific portrayals of interactions within several dream reports.

Hall performed a very informative study of an engineer in his early 30s, Karl, who sent him more than 1,000 dream reports and mailed him detailed answers to his inferences (Domhoff, 1996, pp. 178-181; Hall & Nordby, 1972, pp. 119-126). Once again, Hall's inferences were mostly correct, but it is the mistakes that are useful. First of all, Karl had many highly aggressive dreams, especially toward his father, so Hall inferred that he occasionally provoked fights and harbored generally angry feelings. However, Karl reported that he never engaged in fights and regarded himself as being a friendly, warmhearted, and peaceful person. In this instance, Hall made the opposite type of mistake to the one made with the neurotic patient. With the neurotic patient, Hall used hostile interactions with various women to infer that the dreamer disliked his mother, and he was wrong. With Karl he used hostility toward Karl's father (and mother and wife) to infer aggressive interactions with a wider range of people, and he was wrong again, this time because Karl's dream reports do not reveal that he is in general an angry, violent person. On the other hand, Karl repeatedly told Hall that he harbored great anger toward his father, mother, and wife; therefore, the general point once again is that inferences drawn from the analysis of dream reports concerning continuity with waking thoughts and personal concerns should focus on specific people and not on generalities.

On the basis of these and a few other studies discussed in the synthesis chapter, the continuity hypothesis was updated to include both conceptions and personal concerns, which more firmly links the emphasis on conceptions in Hall's (1953) original formulation of a cognitive theory of dreams to his later discovery of how well the frequency of various elements in a person's dream reports tracks her or his waking personal concerns:

In all, the findings presented in this chapter are good evidence for our claim that the conceptions and concerns found in dream content appear to be the same ones operating in waking life. The waking mind and the dreaming mind seem to be one and the same, which is a strong argument for the idea that there is meaning in dreams. (Domhoff, 1996, p. 189)

Studies beginning in the late 1990s fully demonstrated the new, fully cognitive conception of the continuity hypothesis. The largest and most detailed of these studies is based on 3,115 dream reports written down by a divorced woman ("Barb Sanders") over a period of nearly 37 years. However, the first 106 dreams were written down only sporadically during the first 19 of those years; thus, the analysis is for the most part based on 3,009 dreams documented over a period of 18 years and 4 months. This means that she averaged 3.5 dream reports per week during the later time period, when she had the time and motivation to document every dream she recalled. Therefore, she can be classified as a typical dream recaller. In addition to numerous Hall and Van de Castle (1966) content analyses, the study included interviews with the dreamer and four of her friends. The friends were included to serve as "judge and jury" if there were any differences between the investigator's inferences and the responses by the dreamer (in the event, there were very few in this case; Domhoff, 2003, Chapter 5). This study also reintroduced the idea that there are "discontinuities" in dream reports as well as continuities. However, they were not the kind of discontinuities claimed by the various psychodynamic discontinuity theorists of the past. On the basis of an examination of many of the discontinuities in this dream series, discontinuities were hypothesized to be related to possible figurative constructions, which remained poorly studied as of the end of 2016, and/or to possible cognitive glitches, which remain completely unstudied.

The original Barb Sanders study was later extended in various ways, including the addition of more than 1,000 of her subsequent dreams for some new analyses, but so much has been written about the overall results that little more will be said here (Bulkeley, 2009; Domhoff, 2010a). However, a highly original network analysis comparing her waking social networks and her network of dream characters, based on 4,254 dream reports documented over a 41-year period, is well worth noting (Han, 2014). It revealed that the social networks in her dream reports were small-world networks similar to those found in waking life, but her network of dream characters differed from those in her waking life. The people who were emotionally closest to her in waking life tended to appear in dreams together although they were not in the same social networks in waking life (Han, 2014, pp. 50, 67). These results, which appear to take the continuity hypothesis to a more abstract and quantitative level, along with the other findings based on the Barb Sanders series, including her constant rehashing of past relationships and other disappointments, belie any emphasis on the influence of daily events on dreaming and support the conclusion that dreams are not a mere incorporation of everyday waking activities.

It is also relevant to any discussion of the continuity hypothesis that studies of dream series using 40 standardized word strings (Bulkeley, 2012, 2014; Bulkeley & Domhoff, 2010) have lent independent support to the concept, which answers any criticisms of studies of dream series based on the Hall and Van de Castle (1966) coding system by critics of the cognitive version of the continuity hypothesis (Schredl, 2012, pp. 4-5). To take one good example from a comprehensive blind analysis that led to numerous correct inferences, a young woman who wrote down 223 of her dreams as a teenager, and another 63 in her early years of college, had more frequent (and upsetting) dreams about her parents and brother during her first year or two in college than she previously had. This led to the correct inference that she experienced more worry and stress relating to her family while in college than she previously had. As she explained in response to this inference:

My first semester at college was very difficult and I was extremely homesick. I also had a spike in my anxiety, which led me to seek counseling through the college. I ruminated and worried a lot, missed my family, and was sure that now that I was away at school, something bad would happen. (Bulkeley, 2012, p. 245)

As the investigator concluded, This finding highlights an important point about the continuity hypothesis: "Dreams accurately reflect emotional concerns but not necessarily actual events" (Bulkeley, 2012, p. 248). This researcher uses the word reflect , but the crucial difference from the incorporation theorists is that he stresses the reflection of concerns, not the reflection of various waking events.

Although the original studies that led to the continuity hypothesis were based on individual dream series, an impressive longitudinal study in which the investigators studied the participants in 1991 and 2000-2001 demonstrated considerable continuity between waking well-being measures and aspects of dream content (Pesant & Zadra, 2006). Then, too, in a study comparing 30-day dream diaries from 35 professional musicians and 30 nonmusicians, which also included a daily questionnaire concerning their involvement with music during the day, the musi- cians dreamt twice as often about music as did the nonmusicians, and the frequency of their musical dreams was greater if they became involved in music as young teenagers or earlier. Even more significant in terms of this article, the frequency of their musical dreams did not relate to the degree of their waking musical activity during the days they kept the dream diary; therefore, the musical dreams did not reflect current waking reality (Uga, Lemut, Zampi, Zilli, & Salzarulo, 2006). These two studies are important because they minimize or eliminate any concern that possible selection biases in self-motivated dream series account for the evidence for the continuity hypothesis.

On the other hand, the possibility that evidence for the incorporation of everyday experiences and passing daily concerns into dreams might be found in controlled studies has received no support. This conclusion first of all rests on a series of three studies using both laboratory-collected and home-collected dream reports to compare daytime concerns with dream reports collected that night. In the initial study, eight participants spent 8 nights each in the sleep laboratory and reported dreams from awakenings after the first rapid eye movement (REM) period of the night. The several judges could not match the dream reports with either the general thought samples or the expressions of significant concerns of that day, which the participants provided before falling asleep (Roussy et al., 1996). A second laboratory study involved 13 women participants who reported dreams after awakenings from three of the first four REM periods on each of 4 laboratory nights. Once again, judges were unable to match the dream reports with either waking thought samples or lists of five significant concerns from earlier in the day that were obtained before the participants went to sleep (Roussy, 1998). The third study simulated everyday conditions by asking 13 women students to write down, on 6 separate days, the events of the day before going to bed, and then to document any dreams they recalled upon awakening the next morning. Using a less complex matching task than in the first two studies, 14 independent women judges could only rarely match the thought samples with the dream reports (Roussy et al., 2000). It is also noteworthy in regard to the experiential incorporation hypothesis that two longitudinal laboratory studies of the dreams of children and adolescents found that they rarely dreamed of their most frequent daytime activities, watching TV and reading, writing, and learning in a classroom setting (Foulkes, 1982, 1999; Strauch, 2005).

In concluding this discussion of what the continuity hypothesis actually involves, it is far more specific, empirically developed, and refined over time than is understood by those who claim that the continuity hypothesis is about the influence of events from waking life on dreaming. As shown in this section, the initial studies leading to this new concept were based on personal dream journals, which are regarded as unobtrusive and nonreactive measures by researchers in personality, social, and community psychology (e.g., Allport, 1942; Baldwin, 1942; Johns, Coady, Chan, Farley, & Kansagra, 2013; Schwartz & Sechrest, 2000, pp. xi-xix; Webb, Campbell, Schwartz, Sechrest, & Grove, 1981; Whitley & Kite, 2013). They are more valuable than many experimentally oriented dream researchers seem to appreciate because they have not been influenced by the demand characteristics, expectancy effects, and social desirability effects that were shown decades ago to vitiate the results of many experimental studies to a far greater extent than many current experimentalists seem to remember (e.g., Kihlstrom, 2002; Orne, 1962; Rosenthal & Ambady, 1995; Rosenthal & Rosnow, 1969). That said, this section also has shown that two studies not based on self-motivated dream series support the continuity hypothesis as well (Pesant & Zadra, 2006; Uga et al., 2006).

Looking for Continuity in All of the Wrong Places

Although it was widely known by the late 1990s that the continuity hypothesis is narrowly focused on the continuity between conceptions and ongoing personal concerns in dreams and waking life, this point is completely ignored in the first full-length article that misunderstood its meaning (Schredl, 2003). The problems started in the first sentence of the abstract, which begins with the pejorative "so-called continuity hypothesis" and then proceeds to incorrectly define it, as shown in the following quotation. The second sentence in the quotation then criticizes this inaccurate and newly created definition of the concept as "very broad and vague:"

The so-called continuity hypothesis of dreaming states that waking experiences are reflected [wrong] in dreams. The formulation of the continuity hypothesis is very broad and vague [wrong], however, so that it seems necessary to investigate factors which might affect the incorporation rate of waking-life experiences [which is completely off base and irrelevant because the hypothesis says nothing about 'incorporation']. (Schredl, 2003, p. 26)

The first two sentences of the main body of the article then mistakenly link the origins of the continuity hypothesis to Freud's concept of the "day residue," which concerns the (usually minor) event from the previous day that Freud (1900, pp. 127, 366-368) claimed to be the starting point for every dream:

Already in 1900, Freud (1) reported that day residues (recent waking-life experiences) are common in dreams. This observation and subsequent empirical studies (overviews: 2-5) have led to the so-called continuity hypothesis of dreaming which states that waking experiences are reflected in dreams; in other words: dreaming is in continuity with waking life. (Schredl, 2003, p. 26)

Thus, in the space of just four sentences, the continuity hypothesis has been stood on its head. Dreams do not "reflect" waking-life experiences; for the most part they express (enact, embody, dramatize) personal conceptions and ongoing personal concerns relating to the important people and activities in the dreamers' waking lives. This hypothesis is not "very vague and broad" but was made so by the critic through a mischaracterization of it. Nor did it have its origins in Freud's (1900, p. 127) claims about the day residue; it says nothing about day residues and represents a major departure from the Freudian theory of dreams.

After wrongly summarizing the established continuity hypothesis, the article then proceeds to review the literature on the influence of (a) day residues, (b) the experimental manipulation of presleep events, and (c) laboratory references in dreams collected in sleep laboratories. It also examines (d) the degree to which ordinary daytime experiences are incorporated into dream content. However, all of these alleged incorporations are relatively rare, and some of them are trivial, as overviewed in the next section. In addition, the failure to find connections between the events of the day and dream reports in the three studies discussed at the end of the previous section is not addressed (Roussy, 1998; Roussy et al., 1996, 2000). Instead, one of those three studies is mentioned in a table concerning the frequency of references of the laboratory setting in REM dream reports collected in a sleep laboratory (Roussy et al., 1996; Schredl, 2003, p. 34).

Surprisingly, the critical review does not cite any of the original work on the continuity hypothesis by Hall and his coworkers (Bell & Hall, 1971; Hall & Lind, 1970; Hall & Nordby, 1972). Although it briefly mentions Finding Meaning in Dreams (Domhoff, 1996) as one of four sources providing "overviews" of "empirical studies" (Schredl, 2003, p. 26), it does not indicate that the definition of the concept in that book and the empirical evidence that it cited are completely at odds with its account. Nor is there any mention of the large body of work on dream series that is presented in detail in Finding Meaning in Dreams (Domhoff, 1996). Despite these many problematic aspects to Schredl's (2003) original critical article, there is no criticism of any of the many omissions and distortions in it, or even any mention of them, by any of the authors who had cited it and made use of its redefinition of the continuity hypothesis by the end of 2016 (e.g., King & DeCicco, 2009; Malinowski & Horton, 2011, 2014; Revonsuo et al., 2015; Sándor et al., 2016; Selterman et al., 2014).

In response to criticism of his first analysis of the continuity hypothesis (e.g., Bulkeley, 2011; Domhoff, 2011a), Schredl (2012, p. 1) quotes four paragraphs from the popular book by Hall and Nordby (1972), which he had originally ignored, to claim that the authors "clearly indicate that the continuity hypothesis was defined very broadly and clearly goes beyond the notions of conceptions and concerns suggested by Domhoff as the most important aspects of continuity." Although these four paragraphs include the original psychodynamic definition of the continuity hypothesis that was cited earlier in this article ("The wishes and fears that determine our actions and thoughts in everyday life also determine what we will dream about"), which renders his other (often mistaken) claims irrelevant, the crucial point in terms of making any theoretical advances is that the concept had been refined and improved in the ensuing decades (Domhoff, 1996, Chapter 8), as demonstrated in the previous section. By resorting to a debatable exegesis of an outdated text instead of formulating one or more new alternative hypotheses, such as an experiential incorporation hypothesis, this kind of analysis obscures the fact that the present-day cognitive version of the continuity hypothesis abandons some of Hall's original claims and builds in part on work by other cognitive theorists (e.g., Antrobus, 1978; Domhoff, 1996, pp. 7, 209-212; Foulkes, 1982, 1985; Klinger, 1971).

Schredl (2012, pp. 4-5) goes further astray by criticizing the coding categories for emotions in the Hall and Van de Castle (1966) coding system because they allegedly underestimate the frequency of emotions in dreams and overstate the percentage of emotions that are negative. Although there are strong methodological and empirical reasons for doubting this claim (e.g., Domhoff, 2005, in press, Chapters 1 and 6; Foulkes, Sullivan, Kerr, & Brown, 1988), the important point here is that none of the studies of personal concerns was based on emotion codings. Some of the studies included codings for aggressive and friendly interactions, but aggression and friendliness are not among the five to seven basic emotions that are generally agreed upon by research psychologists (e.g., anger, apprehension/fear, sadness, disgust/contempt, surprise, love, and joy; e.g., Ekman, 1992, 2016; Shaver, Schwartz, Kirson, & O'Connor, 1987). On the basis of these points, it can be seen that any emphasis on the incorporation of waking emotional experiences into dreams is an offshoot of Schredl's (2006, 2012) experiential incorporation hypothesis, which could be called the emotion assimilation hypothesis to distinguish it from the continuity hypothesis (e.g., Malinowski & Horton, 2014, 2016; Schredl, 2006; van Rijn, Eichenlaub, & Blagrove, 2016).

A year after Schredl's (2003) original summary article modifying and criticizing the continuity hypothesis appeared, Schredl and Hofmann (2003) repeated the same inaccurate claims about the definition of the continuity hypothesis in the context of a study that led to many similar studies by several dream researchers. It is based on 2-week dream diaries that were kept by 133 participants (104 women and 29 men). At the end of the 2 weeks, the participants were asked via a questionnaire to estimate the amount of time they had spent during the previous 2 weeks in

a variety of daily activities such as using a computer for working, playing computer games, making telephone calls, spending time with the spouse, reading (divided into leisure time and occupational/studying), driving a car, watching TV, riding a bus/tramway, walking, doing a job, calculating, talking with friends, writing, and being in nature. (Schredl & Hofmann, 2003, p. 301)

It apparently did not seem relevant to these researchers that accurate after-the-fact assessments of the time spent on these activities over a 2-week period would be a considerable feat of memory, although one of the coauthors later acknowledged that this approach "suffers from possible recall problems" (Schredl, 2012, p. 5).

On the basis of 443 dream reports from the 133 participants, the investigators found a range of correlations between the amount of time spent in various waking activities and the frequency with which those activities appeared in the dream sample. They explain their mixed and generally negative findings on the basis of the generality of the continuity hypothesis (without saying that one of the coauthors had created this general, vague, and inaccurate definition) and various methodological problems. They raise these criticisms without even mentioning the unlikelihood that a retrospective questionnaire can provide anything but vague guesswork if modern-day memory research by psychologists has any merit. They then conclude that "the continuity hypothesis in its present general form is not valid [italics added] and should be elaborated and tested in a more specific way" (Schredl & Hofmann, 2003, p. 299).

The conclusion that the continuity hypothesis is not valid is then added to the bill of particulars put forth by dream theorists who focus on the possibility that dreams are a kind of social rehearsal as threat simulations (Revonsuo, 2000) and/or as social simulations (Revonsuo et al., 2015). However, in doing so they ignore the original definition of the continuity hypothesis by claiming it states that dreaming is a "passive mirroring of recent waking life" and therefore can be tested by assuming that "dreams represent a random sample of recent waking experience (or a random sample of their memory representations)" (Revonsuo et al., 2015, p. 10). Nor do the simulation theorists (or the two-dozen other investigators who cite the Schredl & Hofmann, 2003 study) offer the slightest criticism of it, not even for the use of a retrospective questionnaire.

Furthermore, the simulation theorists do not adequately comprehend the implications of the findings on consistency in a person's dreams over months, years, and decades, based on nearly two dozen studies, which reveal that people tend to differentially dramatize (not passively reflect) the same ongoing concerns about interpersonal relationships and favorite avocations on a very regular basis (Domhoff, 1996, Chapter 7; 2010a; Hall & Nordby, 1972, Chapter 5). The detection of repeated patterns, especially in instances involving frequent dreams about failed past relationships, or about deceased love ones, is a major addition to the already strong evidence that calls into question any claim that dreams are primarily a reflection of recent events (Domhoff, 1996, Chapter 7; 2003, Chapter 5; 2015; in press, Chapter 3).

The inadequacies of the Schredl and Hofmann (2003) study, and the simulation theorists' misuse of it, to one side, studies based on retrospective estimates of time spent in waking events continued to be the predominant basis on which Schredl and his coworkers studied experiential incorporations. In a backward step from the Schredl and Hofmann (2003) study, many of these studies did not even involve keeping a dream diary. For example, in a study of 82 truck drivers that was said to support the experiential distortion of the continuity hypothesis, the participants were asked to estimate the amount of time they spent driving a truck each week, the frequency with which they dreamt, and the percentage of their dreams that were about truck driving (Schredl, Funkhouser, & Arn, 2005, p. 181). Another study asked 131 college students, some of who were athletes, to estimate the frequency with which they dreamt, the amount of time they spent playing sports and reading, and the percentage of their dreams that involved sports or reading (Schredl & Erlacher, 2008, p. 268-269). Still another study was said to support the continuity hypothesis because the students who replied on a questionnaire concerning typical dreams that they had killed someone in a dream had higher hostility scores on a personality test (Schredl & Mathes, 2014, p. 178).

In a questionnaire study of music and dreams, 144 students, including music students, estimated the time they spent in music-related activities in waking life and the frequency with which they dreamt about music. The investigators found that "the amount of time invested in music activities during the day is directly related to the percentage of music dreams, thereby confirming the continuity hypothesis" (Vogelsang, Anold, Schormann, Wübbelmann, & Schredl, 2016, p. 132). However, they then note the "possible recall biases regarding retrospective measures" and suggest future studies using dream diaries (Vogelsang et al., 2016, p. 132). Because there is already a dream-diary study showing that the amount of time involved in music during the day did not relate to daily estimates of the frequency of music dreams for musicians (Uga et al., 2006), a study the authors duly cite, it seems unlikely that their music study, based as it is on estimates in response to a retrospective questionnaire, or any of the studies cited in the previous paragraph, can be taken seriously as studies of the continuity hypothesis, even for their version of it.

Questionnaire studies that ask for estimates about dream recall frequency, dream content, and time spent in various waking activities are simply correlational studies of various dimensions of waking thoughts and beliefs. Because very few people have any detailed knowledge of their dreams, studies of the current beliefs that people have about them are probably based on a combination of cultural stereotypes, personality variables, dreams recalled within the previous few days, selective recall of a few memorable dreams, and the whims of the moment (e.g., see Beaulieu-Prévost & Zadra, 2005a, 2005b, 2007, 2015; Bernstein & Belicki, 1996; Bernstein & Roberts, 1995).

The Rarity of Everyday Influences on Dreaming

According to cognitively oriented dream researchers, the appearance of daily events in dreams is a relatively minor matter because dreams are dramatic simulations based on the human ability to imagine (e.g., Antrobus, 1978, 2000; Foulkes, 1985, 1999; Hall, 1991; Reinsel, Antrobus, &Wollman, 1992). Their claims are fully supported by the three careful studies of the issue discussed in an earlier section (Roussy, 1998; Roussy et al., 1996, 2000). They are also supported by the rarity of any evidence for the influence of daily experience on dream content in many other kinds of studies.

For example, in the case of day residue, five detailed studies from decades ago, one of which involved a study of a lengthy dream series that included mentions of day residues, demonstrated that only approximately half of dreams contain even the slightest day residue that can be identified by the dreamer (Botman & Crovitz, 1990; Harlow & Roll, 1992; Hartmann, 1968; Marquardt, Bonato, & Hoffmann, 1996; Nielsen & Powell, 1992).

As far as stimulus-incorporation studies, early laboratory dream researchers found that it is very difficult to influence dream content with either presleep stimuli, such as fear-arousing or sensual movies, or with concurrent stimuli administered during REM periods, such as sounds or the whispering of the names of significant people in the dreamers' lives (e.g., Berger, 1963; Dement, 1965; Dement, Kahn, & Roffwarg, 1965; Foulkes, 1996; Foulkes & Rechtschaffen, 1964). Moreover, the impact of external stimuli on dreams may be exaggerated because the criteria for incorporation were very loose in several studies; sometimes, alleged metaphoric expressions of the stimulus, which are notoriously difficult to validate, were counted as correspondences (see Arkin & Antrobus, 1991, for a critical review).

Suggestions during sleep related to the dreamer's current concerns (which may or may not be very personal or long felt) fared no better in a laboratory study. In this carefully controlled study, seven young adult men each slept in the laboratory for 4 consecutive nights: 1 night for adaptation and 3 for studies of their incorporation of recorded words that were played several times during different REM periods. Some of the stimulus words related to their current concerns, as ascertained via a questionnaire, and some did not (Hoelscher, Klinger, & Barta, 1981, p. 89). Fifty-six of 59 REM awakenings yielded a dream report. Independent judges concluded that there were only five instances of incorporated current concerns, a very meager return (Hoelscher et al., 1981, p. 90).

One exception to these generalizations about the small impacts of external stimuli can be found in a pilot study in which the researcher used a rough equivalent of a blood-pressure cuff to apply increasing pressure during REM periods to the legs of four acquaintances of the researcher who agreed to participate; this somatosensory stimulation was frequently incorporated as simple, direct sensations of pressure or squeezing (Nielsen, 1993). Another exception may concern the influence of the smell of rotten eggs on dream content (Schredl, Atanasova, et al., 2009; Schredl, Hoffmann, Sommer, & Stuck, 2014). Contrary to what these investigators seem to believe, their studies using extraordinary stimuli, which are rarely if ever encountered in ordinary daily life, reveal just how unlikely it is that the events of the day are incorporated into dreams.

On the basis of a consideration of the laboratory dream research literature over a period of nearly 40 years, a dream researcher who did several studies that tried to influence dream content later concluded that

Probably the most general conclusion to be reached from a wide variety of disparate stimuli employed and analyses undertaken is that dreams are relatively autonomous, or 'isolated,' mental phenomena, in that they are not readily susceptible to either induction or modification by immediate pre-sleep manipulation, at least those within the realm of possibility in ethical human experimentation. (Foulkes, 1996, p. 614)

Nor have more recent studies of laboratory incorporations shown any higher percentages, and some of them suggest that personal concerns are the key reason for these few incorporations. The rarity of incorporations, even during the sleep-onset period, is demonstrated in a study in which participants played a computer game ( Tetris ) for 7-9 h a day for 3-4 days in a row (Stickgold, 2003, pp. 10-14). Although 64% of the 27 participants reported at least one incorporation after being "automatically and repetitively prompted for mentation reports during the first hour of attempted sleep," the actual number of incorporations was only 9.8% of all probes for 12 novice players and 4.8% for 10 experienced players; it was 7.2% for 5 participants suffering from profound amnesia, who were included because they might report imagistic evidence of playing the game although they could not remember they had played it (Stickgold, 2003, pp. 10-11). Although the investigator was gratified by the differences among the three groups because he thinks they support his theoretical assumptions, the crucial point as far as this article is concerned is that these figures are reminiscent of the low levels of incorporations found in earlier studies and once again are based on an experimental manipulation that far exceeds the bounds of everyday life (Arkin & Antrobus, 1991).

In a study in which 50 participants took part in a task in which they individually learned how to negotiate a virtual maze on a computer screen and then took a nap, only 4 of them reported thoughts and images after awakenings that related to the task they had learned. Three of them "reported task-related mentation at sleep onset (following at least one minute of continuous sleep), whereas the fourth reported a maze-related dream after awakening from stage 2 sleep at the end of the nap period" (Wamsley, Tucker, Payne, Benavides, & Stickgold, 2010, p. 850). Once again, this is a very meager amount of incorporation for an intense daytime experience (a difficult learning task) that had taken place not long before their naps. Moreover, tasks such as the Tetris game and the virtual maze negotiation may be incorporated on rare occasions because they are "related to concerns about pre-sleep task performance," which is consistent with the original cognitive meaning of the continuity hypothesis (Blagrove, Ruby, & Eichenlaub, 2013, p. 609). Put another way, many people feel personally threatened when they are asked to perform a challenging task.

Similar conclusions seem to follow from a study in which four students (a very small sample size) taking a 6-week second-language immersion class in French each slept in the laboratory 4 times (once before the class began, twice during the 6 weeks of instruction, and once after the class ended). The dream reports mentioning the attempt to learn French portray frustration and failure in grappling with what the person was trying to learn, not a simple incorporation of the participants learning French in the classroom and speaking French in the dormitory and dining hall. As the researchers who performed this work put it, the findings "support the notion that incorporations of learning experiences into dreams that contain frustration and anxiety do not appear to be associated with the learning process [italics added] but instead may be a reflection of obstacles encountered" (De Koninck, Wong, & Hébert, 2012, p. 190). In other words, they are about a frustrating waking concern, a new personal concern they have developed in an unusual and challenging environment in which the expectations as far as learning are very high.

Turning to studies of the appearance of the laboratory setting in dreams collected in the sleep laboratory, Schredl (2003, p. 35) is mistaken in claiming that "the experimental setting strongly affects dream content" and that "participation in a laboratory experiment is 'real' stress." In fact, after one or two adaptation nights, during which portions of the left hemisphere are now known to be more alert than on later nights in a new place (Tamaki, Bang, Watanabe, & Sasaki, 2014, 2016; Tamaki, Nittono, Hayashi, & Hori, 2005), the percentages for incorporation then decline to a low level because the participants know what to expect. However, he may be right that "the incorporation rates are higher than those for experimental manipulation of the pre-sleep situation" (Schredl, 2003, p. 35).

In the best-controlled early study of the effects of the laboratory on dreaming and dream content, it was determined that only 10.6% of the dream reports from 11 college male participants included any portion of text, however brief, that related to the experimental situation. This figure did not vary from the first night they were awakened, after 3 adjustment nights in which they were not awakened, until the end of the participants' involvement in the study; furthermore, only 6.2% of the dream reports contained a significant amount of laboratory-related content, a fact that is also reported in Schredl's (2003, p. 34) assessment of the evidence for everyday incorporations (Hall, 1967, pp. 200, 202-206). Of 32 dream reports with laboratory elements, 13 contained anxiety or hostility in relation to the experiment or the investigators (40.6%), which suggested negative concerns. Surprisingly, the investigator found that the participants who developed the most interest in the study (as evidenced by the questions they asked before and after experimental nights) dreamed most frequently about the laboratory setting, which suggests a positive concern with the study on their part (Hall, 1967, p. 198).

Ironically, this study also provided inadvertent support for the not-yet-formulated continuity hypothesis in that several participants dreamt very frequently about their major waking interests, dwarfing the handful of dreams that included laboratory elements. For example, 30% of the 50 dream reports from a participant preoccupied with sports cars included a sports car; a person focused on a small singing group to which he belonged dreamt about it in 40% of 35 reports; and a person whose chief avocation involved airplanes dreamt about them in 32.3% of his reports (Hall, 1967, pp. 204-205).

Nor is there any evidence that the laboratory situation is continuously stressful even for children ages 4-10, as shown through careful monitoring of their prebedtime behavior and sleep patterns in both longitudinal and cross-sectional studies (Foulkes, 1982; Foulkes, Hollifield, Sullivan, Bradley, & Terry, 1990). The 14 preschool children in the longitudinal study were rated as calm or relaxed on 79% of the nights and as extremely anxious on only 5.4% of the nights. Their median time to fall asleep was 20 min; the same children took only 12 min when they returned 2 years later. The median number of spontaneous night awakenings was 1 or 0, and the median time awake during the night was 8 min or less (Foulkes, 1982, pp. 33-34). In a cross-sectional study involving 80 children within a month of their fifth through eighth birthdays, the participants scored low on a 5-point anxiety scale that was administered on each visit and on average fell asleep 12.5-14.5 min after the lights were turned out (Foulkes et al., 1990, p. 454).

Finally, there are well-designed studies of the degree to which everyday daily life is reflected in dream reports. In these studies the participants kept 1-week or 2-week records of the major daily events, major waking concerns, and personally significant events in their waking lives, as well as of any dreams they recalled. Therefore, these studies may be more accurate than the numerous studies based on retrospective estimates of time spent on a certain activity. However, they involve a different kind of judging task that has its own methodological difficulties, leading to a complex mix of findings that will need to be sorted out in future replication studies before they can be used to balance out the overwhelmingly negative results in relation to the experiential incorporation hypothesis. Specifically, in these studies the participants are asked at the end of the data-collection phase to go through the dream diaries and their record of daily events to make judgments on the correspondences they see between dreams and waking concerns and experiences and whether the concerns and experiences are from the same day as the dream occurred or from any of the days before the dream occurred (e.g., Henley-Einion & Blagrove, 2014; van Rijn et al., 2015).

These studies sometimes show some evidence for the inclusion of recorded events from the same day as the dream (day residue) and for the appearance of recorded events in dreams that occur 5-7 days later (the "dream-lag effect"). However, the results vary depending on whether participants "give a single rating to the degree of correspondence between each dream report and each diary record" or "rated separately the intensity of as many correspondences as they could identify between each dream report and each diary record" (Henley-Einion & Blagrove, 2014, p. 71). In the single-rating design, it is uncertain as to "how participants were making these global judgments"; as for the methodology using multiple correspondences, any statistically significant evidence of incorporations disappears, perhaps because some participants "have an excessive propensity to connect events" (Henley-Einion & Blagrove, 2014, pp. 72, 87).

However, it was then found that there is a statistically significant day-residue effect for the group of participants who identify less than the median number of overall correspondences, but why this would be so is not clear. Thus, this unanticipated finding for below-median responders needs to be replicated, and then further studies would be necessary to understand why low-responding participants would be more accurate than those who provide an above-median number of overall correspondences (Henley-Einion & Blagrove, 2014, p. 72).

In short, methodological issues make it uncertain as to whether statistically significant inclusions of thoughts, events, and experience from daily life have been demonstrated in these more recent and well-designed studies. Moreover, there are some indications that any incorporations that do occur in these more recent studies lend support to the cognitive version of the continuity hypothesis. In one of the most complex and best-controlled studies of this type, based on both laboratory and home-collected REM dream reports, the presence of the dream-lag effect "is dependent on the salience or personal importance of waking life events" (van Rijn et al., 2015, p. 107). This type of finding is also reported in a study of 2-week dream diaries and daily events diaries from 32 participants by two researchers who adhere to the emotion assimilation hypothesis; they conclude that "major daily activities were included significantly less than the combination of personally significant experiences, major concerns, and novel experiences" (Malinowski & Horton, 2014, p. 31).

As this overview shows, these studies reveal that daily events and experimental manipulations have very little influence on dream content. Metaphorically speaking, it is as if highly sophisticated statistical investigators reported after performing an exhaustive multivariate regression analysis that they had explained approximately 5-7% of the variance. However, if the most recent incorporation studies (Henley-Einion & Blagrove, 2014; Malinowski & Horton, 2014; van Rijn et al., 2015) can be replicated, then the small number of personally significant experiential incorporations would be a useful augmentation to the version of the continuity hypothesis that is consistent with recent theorizing by cognitive psychologists (Domhoff, 2003, in press; Foulkes, 1985, 2017; Foulkes & Domhoff, 2014).

As this article fully documents, the continuity hypothesis is a precisely defined cognitive concept focused on a person's conceptions and concerns, with concerns referring to a person's primary personal interests and preoccupations of both a positive and negative variety, many of which may extend back to their college years in the case of some adults that have been studied (Domhoff, in press, Chapters 3-4). As such, this concept is an important part of the effort to develop a neurocognitive theory of dreams that envisions dreaming as a form of spontaneous thought, far removed from the theoretical tradition of stimulus-response psychology, a tradition that naturally leads to a focus on the incorporation of events from daily life (e.g., Antrobus, 1978; Antrobus et al., 1970; Domhoff, 1996, pp. 210-212; 2010b; Domhoff & Fox, 2015; Foulkes, 1985, 1999; Foulkes & Domhoff, 2014; Fox et al., 2013; Reinsel et al., 1992). Therefore, the continuity hypothesis should be distinguished from the broadly defined "experiential incorporation hypothesis" created by Schredl (2003) and endorsed by many others (e.g., Malinowski & Horton, 2014; Revonsuo et al., 2015; Sándor et al., 2016; Selterman et al., 2014), which encompasses day residues, stimuli-induced incorporations within the laboratory setting, incorporations of the laboratory setting into dream reports collected in a sleep laboratory, and the appearance of everyday experiences from recent daily life.

Because of the very different theoretical orientations and methods that experiential incorporation theorists (e.g., Schredl, 2003; Schredl, 2012) and social rehearsal/simulation theorists (Revonsuo, 2000; Revonsuo et al., 2015) use in studying the relationship between waking and dreaming, and out of respect for the difficulties of developing a new theory in any research field, these dream researchers should adopt another label for the hypothesis they favor over the cognitive version of the continuity hypothesis. The concept of a continuity hypothesis owes its provenance to the cognitive tradition of dream researchers, who are trying to develop a new theory of dreams; therefore, the concept should not be redefined and diluted, which generates lack of clarity and confusion. The cognitively oriented dream theories are as independent from the psychodynamic discontinuity theories of the prelaboratory era as they are from the experiential incorporation and emotion assimilation hypotheses that flourish at the present time.

It may seem unusual that a dream researcher whose work is being critiqued in this article would be a coauthor of an article that directly states that his view of the continuity hypothesis is incorrect. However, he is a coauthor on that 2005-2006 article because he kindly provided the set of dream reports that is the basis for the empirical analysis in that article. Although he had the opportunity to read and comment on a draft version of it, he did not comment. The critique of the emphasis on experience in the article reads as follows, with the italics in the original published article:

More theoretically, the comparison of dream content with waking life suggests that dreams express our conceptions of the people and activities that concern us in waking life (Domhoff, 2003), not merely our experiences in waking life. This distinction between conceptions and experiences is a crucial one because Hall and Nordby's (1972) evidence and line of reasoning lead to a cognitive theory of dreams, whereas an emphasis on experiences in waking life implies a more environmentalistic or behavioristic theory. (Domhoff et al., 2006, p. 277)

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Acknowledgements

The author thanks Richard L. Zweigenhaft for his critical reading of the first draft of this article, which led to several changes; to Kelly Bulkeley for very helpful suggestions to make the manuscript more focused; and to David Foulkes for two readings of subsequent drafts of the article, which led to even more changes.

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Investigation on Neurobiological Mechanisms of Dreaming in the New Decade

Serena scarpelli.

1 Body and Action Lab, IRCCS Fondazione Santa Lucia, 00179 Rome, Italy; [email protected] (V.A.); [email protected] (L.D.G.)

2 Department of Psychology, Sapienza University of Rome, 00185 Rome, Italy; [email protected] (M.G.); [email protected] (A.M.G.)

Valentina Alfonsi

Maurizio gorgoni, anna maria giannini, luigi de gennaro.

Dream research has advanced significantly over the last twenty years, thanks to the new applications of neuroimaging and electrophysiological techniques. Many findings pointed out that mental activity during sleep and wakefulness shared similar neural bases. On the other side, recent studies have highlighted that dream experience is promoted by significant brain activation, characterized by reduced low frequencies and increased rapid frequencies. Additionally, several studies confirmed that the posterior parietal area and prefrontal cortex are responsible for dream experience. Further, early results revealed that dreaming might be manipulated by sensory stimulations that would provoke the incorporation of specific cues into the dream scenario. Recently, transcranial stimulation techniques have been applied to modulate the level of consciousness during sleep, supporting previous findings and adding new information about neural correlates of dream recall. Overall, although multiple studies suggest that both the continuity and activation hypotheses provide a growing understanding of neural processes underlying dreaming, several issues are still unsolved. The impact of state-/trait-like variables, the influence of circadian and homeostatic factors, and the examination of parasomnia-like events to access dream contents are all opened issues deserving further deepening in future research.

1. Introduction

Dreaming, or sleep mentation, is an intriguing experience occurring during any human sleep stages [ 1 ]. It can happen many times per night and is characterized by different degrees of emotional intensity, bizarreness, visual vividness, and narrative complexity. Although dreaming is impossible to study directly, many researchers investigated its neural bases in healthy subjects [ 2 , 3 , 4 , 5 , 6 , 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 ] or clinical samples [ 15 , 16 ] in different age ranges [ 10 , 17 , 18 ]. Most investigations focused on dream recall (DR), namely the individuals’ content reported immediately after awakenings. However, several methods were applied to study dreaming, from naturalistic investigations using perspective diaries or retrospective questionnaires to psychophysiological and neuropsychological approaches.

The first studies on neural bases of dreaming were carried out on brain-lesioned patients to understand anatomical correlates of dream disappearance or excesses [ 19 ]. Overall, these early findings suggested that two brain networks are engaged in the oneiric processes: a posterior and an anterior system. Specifically, lesions concerning the right and unilateral posterior networks can affect some dream aspects, namely color or motor features [ 19 ]. Temporo-parietal junction (TPJ)-responsible for visual imagery during the waking state- is also involved in this system.

A lesion over the bilateral anterior system causes dream cessation, i.e., the so-called “anoneria” or Charcot–Wilbrand syndrome [ 19 ]. The ventromedial prefrontal cortex is included in this network and is involved in mental representation in wakefulness. In addition, many of the afferent and efferent fibers of these regions are linked to the limbic system, recognized as essential for dreaming [ 20 ]. Prefrontal leucotomy is also linked to dream loss, while lesions of the medial prefrontal cortex (mPFC) result in excessive dreaming, such as the so-called “anoneirognosis” [ 19 ].

These neuropsychological observations on brain lesioned patients enhanced our knowledge about the structures involved in dream experience. However, these studies are not free from flaws because (a) DR in patients could be affected by pharmacological treatments; (b) DR was retrospectively collected; (c) no information was available about DR during premorbid conditions [ 21 ].

Moreover, the research on the neural bases of sleep mentation is always interwoven with the issue concerning the functional role of dreaming. Oneiric experience has been considered a simulation of reality offering a coping strategy to face daily adverse events [ 22 ]. More in general, it could represent an emotional regulator and some neurobiological correlates provide support to this view [ 23 ]. In this vein, some authors highlighted that daily experience could be incorporated and re-organized during dreaming. For instance, pre-sleep rehearsal of actual concerns and problems before sleeping can appear in DR [ 24 , 25 ].

New techniques have been introduced to obtain a deeper understanding of the neurobiological processes underlying dreaming starting from these early studies. Furthermore, in a pioneering way, various studies were aimed to modulate dream experience, influencing it by brain stimulation techniques (e.g., [ 26 ]).

Considering this background, here, we intend to summarize the available findings on the neural basis of dreaming with a focus on the most recent studies supporting the two major hypotheses about the production, elaboration and recall of dream experience. Further, we provide a comprehensive update about the findings resulting from stimulation protocols aimed to manipulate sleep mentation. Finally, based on the available empirical evidence, we intend to propose promising directions for future studies on dream experience.

2. Overview of Current Knowledge

The neuroscientific study of dreaming has undergone remarkable advances thanks to different techniques of investigation: (a) Electroencephalographic (EEG) and polysomnographic (PSG) protocols, or video-PSG recordings with provoked awakenings; (b) intra-cranial/stereo-EEG (iEEG); (c) functional neuroimaging, such as Positron-Emission Tomography (PET), Functional Magnetic Resonance Imaging (fMRI); (d) structural neuroimaging, such as MRI and MRI associated with Diffusion Tensor Imaging (DTI).

2.1. Continuity Hypothesis

The original “continuity hypothesis”, proposed in the early 1970s [ 27 ], posited for the first time that personal concerns and conceptions characterizing the waking thoughts have their own continuity in sleep [ 28 ]. Dream experts broadly discussed this assumption, and many investigations were aimed to test it, considering different aspects of mental activity during sleep. Several re-interpretation and adjustments were made to the continuity hypothesis. Indeed, many researchers concluded that dreams reflect the waking-life and stated that dream contents could represent the incorporation of daily experience. In particular, studies suggested that the most emotionally intense waking events were incorporated during mental sleep activity [ 29 ]. More directly, the incorporated memories of waking events occurring 1–2 days before the dream were defined as “day-residue effect”, while life experiences related to 5–7 days before the dream were called “dream-lag effect [ 30 ]. This phenomenon was mainly found in association with Rapid-Eye Movement (REM) sleep dreams [ 31 ]. In this respect, some authors hypothesized that the dream-lag effect might represent the transfer of new memory representations from the hippocampus to the neocortical circuits, providing a gradual integration into older representation [ 32 ]. Specifically, the dream-lag effect could be the expression of emotional memory processing during REM sleep [ 23 , 33 ].

This evidence assumes that dreaming and memory processes are interrelated. In this view, some studies started from the idea that dream recall (DR) upon awakening could be considered a peculiar form of declarative memory and, more specifically, of episodic memory [ 3 , 5 , 8 ]. PSG with provoked awakenings is the gold standard method to study DR [ 34 ]. At the beginning of the 21st century, several studies started to investigate EEG activity preceding the DR to identify specific patterns of activity that could predict the presence of dream experience [ 2 , 3 , 4 , 5 ]. Convergent findings pointed to the relationship between frontal theta oscillations (5–7 Hz) and DR upon awakening from REM sleep [ 5 , 8 , 10 ]. Moreover, the frontal theta activity during REM sleep seems to be a predictor of recent waking-life experiences [ 29 ]. Additionally, a decreased alpha activity (8–12 Hz) over the parieto-occipital region could be a predictor of DR from stage 2 non-REM (NREM) sleep [ 3 , 5 ]. Partly in line with this pattern, Takeuchi et al. [ 2 ] found that recall condition was predicted by reduced alpha and sigma power in central areas during NREM sleep onset periods. Nevertheless, they also revealed that higher alpha and sigma (12–15 Hz) power in the same regions were related to DR during REM sleep onset [ 2 ].

It should be considered that both alpha and theta activity are linked to mnestic neural processes during wakefulness [ 35 ]. Specifically, increased frontal theta oscillations and reduced parieto-occipital alpha activity predict good performances in episodic memory tasks during the waking state [ 36 ]. Consistently, an iEEG study on patients with drug-resistant epilepsy showed that DR was associated with the medial temporal lobe activity, which is strongly implicated in memory processes [ 37 ]. Specifically, the authors found that high recallers showed enhanced rhinal-hippocampal and intrahippocampal EEG coherence in all frequency bands (especially in the theta band), underlining that anatomic and functional connectivity between limbic structures was crucial both for waking declarative memories and DR [ 37 ].

In light of this, the continuity hypothesis could be extended to a neurobiological perspective. In fact, these patterns are responsible for the encoding and retrieving episodic memories in both sleep and waking-life, indicating the existence of shared mechanisms for cognitive elaboration across different states of consciousness [ 38 ].

In this regard, neuroimaging studies found that specific structures involved in cognitive and emotional processes during wakefulness are activated during sleep. It should be noted that most authors focused on REM sleep, according to the traditional belief that dreaming is an epiphenomenon of this sleep stage [ 39 ]. Specifically, studies by means of PET and fMRI highlighted some REM sleep neural correlates overlapping to those of dreaming. These functional neuroimaging studies demonstrated that limbic and paralimbic structures, thalamus, basal forebrain, pontine tegmentum are significantly activated during REM Sleep [ 40 , 41 , 42 ].

It is well-established that amygdaloid complexes, hippocampal formation and anterior cingulate cortex have a pivotal role in emotional memory encoding and consolidation. During REM sleep, their activation may explain the emotional load (i.e., emotional intensity) of some dream contents [ 40 , 41 , 42 , 43 ]. Although this sleep stage is characterized by muscle atonia [ 44 ], neuroimaging studies found higher activation over motor and premotor regions, consistently with the fact that dreamers often are engaged in motor behaviors [ 40 , 45 , 46 ].

Conversely, other regions appear to be hypoactivated compared with the wakefulness, such as the precuneus, orbitofrontal cortex, posterior cingulate gyrus and dorsolateral prefrontal cortex [ 41 , 42 , 43 ]. The reduced activation in these areas is related to the deficit in executive function, time perception and insight during dreaming [ 43 ].

Most of the mentioned findings stem from protocols that did not actually collect dream reports after awakening, based on an ideal correspondence between REM sleep and dream generation. Structural brain imaging studies overcame this assumption and focus on morphoanatomical parameters (e.g., volumetric measures) and interindividual variability of dream features.

For example, DTI is an MRI technique that examines the white matter integrity using anisotropic diffusion to estimate the neuroanatomical organization of the brain [ 47 ]. Employing this approach De Gennaro et al. [ 6 ] found that volume and diffusivity (magnitude of neuronal water diffusion) of the hippocampus and amygdala correlated with qualitative dream features. Specifically, left amygdala volume was related to the bizarreness of dreams, while a smaller volume of the left hippocampus and a larger volume of the right hippocampus was linked to emotional load. Additionally, the lower structural integrity of the left amygdala was associated both with reduced emotional load and shorter dream reports [ 9 ]. More recently, an MRI study on patients affected by Parkinson’s disease (PD) replicated these findings [ 15 ]. Measures of subcortical volumes and cortical thickness also revealed that visual vividness is related to the amygdalae and the thickness of the left medial prefrontal cortex (mPFC). In addition, the emotional intensity was positively correlated with hippocampal volume. PD patients under pharmacological treatment can also be considered a model to study the dopaminergic system involvement in dreaming. In other words, a higher dosage of dopamine agonists indicated a hypodopaminergic state. Authors reported that dopamine dosage negatively correlates with dream bizarreness and emotional load in patients with PD [ 15 ]. On the one hand, these findings confirmed that the mesocortical–mesolimbic dopamine system has a crucial role in dream phenomenology, as suggested by Solms’ observations [ 19 ]. On the other hand, the evidence of the mPFC and dopamine level contribution in qualitative aspects of dreaming supports the neurophysiological continuity-hypothesis, given the well-established role of the mesolimbic-dopaminergic system in modulating reward and motivational behaviors [ 48 ].

Interestingly, these findings pointed to the existence of specific stable characteristics of brain structures impacting on the qualitative features of dream reports.

In keeping with these studies, another research group tested the interindividual differences between people with higher dream recall rate (High Recallers, HR) and low recall rate (Low Recallers, LR). Differences between HR and LR were found both during wakefulness and sleep [ 7 , 49 , 50 ]. Eichenlaub et al. [ 7 , 49 ] using PET highlighted that HR increased cerebral blood flow than LR in the mPFC and temporo-parietal junction (TPJ) during REM sleep, N3 and wakefulness [ 49 ]. More recently, the authors evaluated the association between brain anatomical structures and dream recall rate, focusing on mPFC, TPJ and limbic structures (amygdala and hippocampus) [ 51 ]. They showed significant differences between groups in the white matter of the mPFC. In addition, the authors confirmed no differences in the amygdala and hippocampus concerning dream frequency, confirming previous results [ 6 , 15 ].

Overall, the findings suggest that amygdala and hippocampus have a parallel role in processing emotional memories during both wakefulness and sleep. The involvement of these subcortical nuclei in dream affect has also been confirmed by two recent studies on brain-damaged patients [ 52 , 53 ]. Specifically, the analysis of dreaming collected from subjects with bilateral amygdala lesions showed that patients had more pleasant and significantly shorter dream reports than control subjects [ 52 ]. Accordingly, these patients also had impaired emotional episodic memory during waking state [ 52 ]. Another study on subjects with bilateral hippocampal damage found that patients had lower DR frequency and less episodic-like contents [ 53 ].

Moreover, converging results provide evidence in favor of the role of mPFC in dream production and recall [ 19 ]. The frontal EEG theta power associated with DR [ 5 , 8 , 10 , 34 ] can be the expression of the involvement of mPFC in the oneiric activity.

It should be noted that the mPFC, along with the TPJ, is part of the Default Mode Network (DMN), the neural circuits underlying resting state, internally oriented mental processes (e.g., mental imagery, mind-wandering) and episodic memory retrieval [ 43 , 54 , 55 ]. Both mind-wandering (or daydreaming) and dreaming include thoughts, sensations, visual-imaginative elements and are characterized by a relative lack of meta-awareness [ 56 ]. According to the authors hypothesized that dreaming is an intensified expression of daydreaming [ 54 ], we point out that these phenomena share the same neural correlates suggesting, once again, a certain continuity between physiological/neurobiological mechanisms subserving cognitive processes in sleep and wakefulness.

Based on this view, some authors combined fMRI with machine learning models with the aim to assess the similarity between waking visual experiences and dreaming including visual elements [ 57 ]. They applied decoding models trained on stimulus-induced brain activity in visual areas during wakefulness and showed an accurate categorization and detection of mental contents. Specifically, they used statistical decoders trained to predict categorical labels of viewed objects/scenes. The target categories to be decoded from brain activity were constructed from dream contents consisted of 20 object categories for each subject. The results revealed that objects or scenes visualized during mental sleep activity could be predicted from specific brain activity patterns. These findings suggested that visual dream contents are represented by discriminative brain activity patterns similar to perception in the visual cortex [ 57 ].

In addition, the relation between waking visual experience and dreaming was showed from previous iEEG study across the medial temporal lobe and neocortex during sleep, wakefulness and visual stimulation with fixation [ 58 ]. By using 72 depth electrodes in neurosurgical epilepsy patients, the authors had the great advantage to provide anatomically and temporally accurate information about the relation between rapid-eye movements (REMs) and underlying activity in visual regions. In fact, REMs onsets are associated, during sleep and waking state, with specific intracranial potentials that resemble ponte-geniculate-occipital waves [ 58 ]. This study reported that single neurons in the medial temporal lobe showed lower firing rates prior to REMs and increasing rates immediately after REMs, similarly to the pattern upon image presentation during fixation in waking state. More directly, the “scanning hypothesis” suggests that REMs during sleep imitate the scanning of dream scenes [ 59 ]. Consistently, a study on patients with REM Behavior Disorder, RBD (i.e., a sleep disorder characterized by intermittent loss of normal muscle atonia REM sleep with the appearance of complex motor behaviors associated with dream mentation) found a substantial concordance between the directions of REMs and the dream scenario with goal-oriented action during RBD [ 59 ]. Consistently, a recent study by Laberge et al. [ 60 ] provided an argument that visual imagery during REM sleep is similar to visual perception during wakefulness. The authors examined smooth pursuit eye movements during tracking of a slow-moving visual target during REM lucid dreaming and showed highly similar smooth pursuit tracking during both waking perception and REM sleep [ 60 ].

Overall, this “brain reading” approach suggests that specific visual experiences during sleep are predicted by the same brain patterns of stimulus perception. In other words, the results support the hypothesis of common neural substrates for waking perception, imagery and visual dream experience. However, it should be noted that the observation by Horikawa et al. [ 57 ] was limited to the wake–sleep transition, and this implies that they studied a dream-like state characterized by hypnagogic hallucinations.

2.2. Activation Hypothesis

Besides the idea that different states of consciousness share similar neurobiological mechanisms, many findings converge to the hypothesis that also the brain activation level during sleep promotes dream recall (DR). Actually, Koulack and Goodenough by their “Arousal-retrieval Model”, posited that the mental activity during sleep could be memorized only when sleep becomes fragmented and short awakenings occur, allowing the dream material to be elaborated and transferred from short-term memory storage to long-term storage [ 61 ].

Hobson and McCarley [ 62 ], shortly after, proposed a neurophysiological model of dreaming: the Activation-Synthesis hypothesis. The model started by assuming that there is an isomorphism between simultaneous physiological and psychological events during sleep mentation. They postulated that dreaming resulted from the periodic activation of the forebrain during sleep. The authors also introduced for the first time the idea that EEG desynchronization may be involved in oneiric processes [ 62 ]. However, it should be noted that all these considerations were based on REM-dreaming equivalence.

In the 1990s, Antrobus introduced the “Activation Model”, suggesting a link between dreaming and local periodic activations during sleep of the same cerebral structures designated for perception, motor and cognitive elaboration during wakefulness [ 63 ].

In keeping with this background, more recently, some studies found that DR was hampered when sleep was characterized by a great amount of slow wave sleep (SWS) and lower awakenings (e.g., [ 64 ]). Subjects with high rates of DR showed increased intra-sleep wakefulness [ 50 , 65 ]. In addition, sleep fragmentation in older people was also related to qualitative aspects of DR. Indeed, the wakefulness after sleep onset during undisturbed night was correlated with high visual vividness of dream reports [ 18 ].

Along the same vein, subjects did not recall dream experience during a recovery night after sleep deprivation. De Gennaro et al. [ 64 ] showed that the greater proportion of SWS—characterized by higher slow wave activity (SWA; 0.5/1–4 Hz)—and the reduced percentage of REMs during recovery night were related with a remarkable difficulty in DR after morning awakening. This could represent a consequence of the elaboration of reduced “dreamlike” materials over the night. It should be noted that SWA decreases across the night as a function of homeostatic sleep pressure and this could explain why dreams are more likely recalled in the early morning hours, sometimes making NREM sleep dream reports indistinguishable from REM sleep dream reports [ 66 , 67 ].

More directly, EEG investigations revealed the link between specific topographic patterns and dream experience [ 4 , 9 , 11 , 12 , 13 , 14 , 18 ]. Overall, the EEG findings point out that the local presence of slow waves in certain regions may interfere with the encoding and recall of dream experience. Zhang et al. [ 11 ] reported that lower global slow oscillations (<1 Hz) were associated with the successful retrieval of dream contents. Additionally, DR would appear when the sleep EEG during NREM sleep was characterized by lower SWA (0.5/1–4 Hz) in the centro-parietal area [ 4 ], in the left frontal and temporo-parietal regions [ 9 ], and in the parietal region [ 18 ]. Siclari et al. [ 13 ] found that dream experience was related to reduced SWA over parieto-occipital regions both during REM and NREM sleep. In parallel, the brain activation expressed in term of higher rapid frequencies (>25 Hz) predicted DR when localized in the parieto-occipital areas during NREM sleep and in frontal and temporal regions during REM sleep [ 13 ]. Similarly, higher beta power (16–24 Hz) is related to DR after REM sleep over the occipital area in a multiple awakenings protocol [ 12 ]. However, it should be mentioned that a recent investigation by Wong et al. [ 68 ] using a serial awakening paradigm failed to find any difference between the recall and non-recall condition.

Interestingly, investigations on clinical samples support the activation-hypothesis. Patients with insomnia disorder reported greater DR frequency [ 69 ]. Consistently with the Arousal-retrieval Model [ 61 ] the relation between insomnia and DR rate may be explained by the amount of awakenings during the night [ 69 ]. Moreover, it should be considered that insomnia patients showed a reduced delta/beta ratio, which had proved to be reliable index of brain activation [ 70 , 71 ]. Although no direct studies have been carried out on EEG correlates of DR in insomniacs, it may be hypothesized that the presence of higher proportion of fast-frequency EEG activity could facilitate DR in these patients. Additionally, a recent study on the neural correlates of dreaming in narcoleptics partly confirmed the role of EEG desynchronization in REM and NREM sleep in promoting DR [ 16 ]. D’Atri et al. [ 16 ] found that recall condition was associated with lower values of delta/beta ratio compared with non-recall. In particular, lower delta power in centro-parietal areas during both sleep stages, and higher beta power in the same regions during NREM predicted DR [ 16 ]. Similarly, narcoleptic subjects reporting lucid dreams (i.e., the experience of being aware of dreaming during sleep) showed lower delta activity during REM sleep than individuals without lucid dreams [ 72 ]. This finding suggests that a more activated brain may represent a privileged context to promote cognitive processing.

Overall, many studies identified the activation over posterior and anterior regions, confirming, again, that they are involved in DR, consistent with earlier studies [ 19 ].

3. Manipulating Dream Experience

3.1. sensory stimulation and dreaming.

Thus far, we have looked at the available studies that made an effort in order to understand the neural correlates of sleep mentation. However, it should be mentioned that, in parallel, multiple older investigations were aimed to directly influence the ongoing dream scenario by using different kind of stimulations, i.e., visual, auditory, somatosensory or olfactory cues.

The first attempt to manipulate dreaming with pre-sleeping external stimuli was reported by Dement and Wolpert [ 73 ]. The authors deprived subjects of fluids one day prior to sleeping and obtained 5 out of 15 REM dreams, including thirst-related content. Goodenough et al. [ 74 ] used stressful films during a pre-sleep period, demonstrating that visual stimulus can increase dreams characterized by negative emotional tone. Moreover, subjects exposed to visual inverting prisms recalled more vivid dreams [ 75 ].

Visual stimuli were also applied during sleep. Rechtschaffen and Foulkes [ 76 ] presented some images during REM Sleep, while subjects’ eyes were taped open. Nevertheless, no incorporation was found.

Concerning auditory stimuli, Berger [ 77 ] found that presenting personally significant names during REM sleep provoked a high rate of indirect incorporation of these stimuli into dream experience. For instance, the names were modified for assonance or in an associative manner [ 77 ]. Similarly, Hoelscher et al. [ 78 ] revealed that words with a relevant personal meaning were more frequently incorporated than other words.

Visual and auditory modalities were combined in a study that applied stimulation both during REM and stage 2 NREM sleep [ 79 ]. Interestingly, the effect was found only when the stimulation occurring in NREM sleep, after which people reported dream with more visual contents [ 79 ]. Although the authors suggested that REM sleep was not very sensitive to visual stimuli, some researchers investigated lucid dreaming, positing that visual stimuli could be incorporated into the dream experience [ 80 ]. Indeed, a study in which participants were requested to wear a LED light-fitted mask detecting rapid eye movements and sending visual stimuli showed that sleeping subjects realized that they were currently in REM sleep stage, increasing the possibility of producing lucid dreams. Participants frequently reported a dream scenario characterized by illuminated environments or containing street lamps [ 80 ].

It should be noted that researchers also shaped dreaming by introducing specific bodily stimulus during sleep. For instance, a spray of water on the subjects’ skin was applied [ 73 ], as well as a thermal stimulation [ 81 , 82 ]. Specifically, Dement and Wolpert by using various external stimuli (i.e., 1000-cps tone stimulus, light flashes, water spray and arousing bell) did not show remarkable effects in modifying the dream content, nevertheless cold water appeared more effective than other stimuli [ 73 ]. Some authors also tried to stimulate subjects with vestibular stimulation, for instance asking subjects to sleep in hammocks [ 83 ]. Other studies evaluated the effect of tactile stimulation by a pressure cuff to a leg, both during REM [ 84 , 85 ] and at sleep onset (stage 1 NREM sleep; [ 86 ]) provoking high incorporation rates, with a percentage of 40%–80%. Furthermore, the incorporation of somatosensory stimuli was obtained by electrical pulses on the wrist to stimulate muscle contractions during REM sleep [ 87 ]. Overall, these kinds of stimulations increased vividness and higher bodily sensations in the oneiric contents.

More recently, olfactory stimulation was successfully used to modulate dreams’ emotional tone during REM sleep [ 88 ]. Indeed, the authors showed that pleasant scent (e.g., rose) were associated with positive dream experience, while unpleasant smell (e.g., eggs) with more negative dream experiences [ 88 ]. Furthermore, it was demonstrated that using an odor presented during REM sleep and previously associated with an image during waking state induced subjects to report a dream experience containing the related image [ 89 ].

The findings of the impact of external stimuli on dream contents are mixed, and no compelling explanation about these phenomena was reported. In fact, the results on dream incorporation are discordant between studies and/or stimuli often fail to be incorporated in the content of dreams directly [ 1 , 76 , 77 ]. Moreover, the method used to classify the incorporation is quite different among studies. The judgment of whether a stimulus had been actually represented into the dream scenario or not was often subjective (e.g., [ 73 ]) and only few studies tried to assess, by an objective method, the presence/absence of incorporation and themes/emotional contents of dreams by identifying defined categories and requiring rating to two independents judges (e.g., [ 77 , 89 ]). However, a systematic and reliable method to study the incorporation is still lacking.

To some extent, these studies showed that the sleeping brain is able to perceive and process much information integrating them in an oneiric narrative. However, some authors suggested that stimuli incorporation into dreams cause a sort of micro-arousals that slightly awaken the sleeping subject so that they perceive it, but not enough to wake them up [ 90 ]. This view seems to be relatively compatible with the activation-hypothesis, however only scarce evidence about neural correlates is available.

3.2. Brain Stimulation and Dreaming

A very innovative line of research is aimed to manipulate dreaming by modulating brain activity directly. This method is based on non-invasive brain stimulation techniques and may be considered complementary to the earlier mentioned protocols influencing dream contents.

Specifically, Transcranial Direct Current Stimulation (tDCS) can induce focal changes in cortical excitability by a constant low-intensity current [ 91 ]. It is well-established that during wakefulness tDCS can alter the activation of motor [ 92 ], somatosensory [ 93 ], prefrontal [ 94 ] and visual cortices [ 95 ]. Additionally, tDCS can impact on cognitive functions, namely working memory [ 96 ] or tactile perception [ 97 ]. In recent years, a growing number of studies have investigated the effects of these techniques in the context of sleep and vigilance [ 98 , 99 , 100 , 101 , 102 , 103 ].

Starting from this knowledge, some investigations assessed tDCS effects on dreaming, intending to explore the involvement of different cortical areas in the oneiric experience. In a pioneering study, the stimulation was applied during Stage 2 NREM sleep over frontal (cathodal) and right posterior parietal (anodal) cortex [ 104 ]. The authors found a greater number of visual imagery reports after tDCS than sham condition, but the same was revealed for the two control conditions (i.e., reversed polarity and other-cephalic tDCS). Hence, the effects seem to be independent of tDCS polarity and the higher visual dream frequency appears to be elicited by a general arousing effect, rather than depending on the specific stimulation of cortical areas [ 104 ]. Moreover, no effect was observed during REM sleep or SWS [ 105 , 106 ]

Bihemispheric tDCS has been recently applied during REM sleep to investigate the potential interference of tDCS over the sensorimotor cortex with movement and bodily sensations of dream experience [ 107 ]. The study showed that subjects awakening after stimulation had a lower rate of reports including movements (especially repetitive actions), while tactile and vestibular sensations were not affected by tDCS. Hence, the authors suggested that sensorimotor cortex is responsible for generating dream movement, confirming that the neural bases of peculiar dream contents are shared between sleep and waking state.

The absence of awareness, and the lack of voluntary control during dream experience are both intriguing issues for dream scientists. In this regard, the researchers started becoming interested in modulating the level of consciousness during sleep, namely, inducing lucid dream experiences [ 26 , 108 , 109 ]. Hobson et al. [ 110 ] proposed that lucid dreaming may stem from the reactivation of dorsolateral prefrontal cortex that, as mentioned, appear deactivated during REM Sleep [ 41 , 42 , 43 ]. In this vein, Stumbrys et al. [ 108 ] applied tDCS stimulation over the dorsolateral prefrontal cortex during REM sleep and observed a small increase of self-reported lucid dreaming only in frequent lucid dreamers.

Further, transcranial alternating current stimulation (tACS) was applied to induce dream lucidity [ 26 ]. tACS exerts its effects by the application of an alternating current to the scalp at a specific frequency. The alternating stimulation drives the cortical network to oscillate at the given frequency, inducing periodic shifts in the transmembrane potential of underlying neurons [ 111 ]. Bearing in mind the involvement of frontal areas in dream activity [ 19 ], Voss et al. [ 26 ] used tACS for 30 s at several different frequencies over fronto-lateral regions during REM sleep. Subjects reported lucid dreams when 25 and 40 Hz stimulation were applied. Conversely, a more recent study on lucid dreaming did not support this evidence, observing no effect of 40 Hz frontal stimulation [ 109 ].

To our knowledge, the only one study using Transcranial Magnetic Stimulation (TMS) tried to manipulate dream experience targeting the posterior parietal cortex. TMS was applied during NREM sleep before awakening subjects and asking them about the presence/absence of dream experience [ 112 ]. The results showed that TMS induces an EEG response characterized by a larger negative deflection (similar to a larger NREM sleep slow wave) and a shorter period of phase-locking associated with non-recall compared to recall conditions [ 112 ]. Additionally, the amplitude of this deflection was negatively correlated with the total word count of dream reports. This response is typical of cortical circuits in condition of bistability between depolarized up-states and hyperpolarized down-states [ 113 ]. Notably, changes in the bistability of cerebral networks, mainly expressed in the form of slow oscillations, modulate the level of consciousness and the ability of the cortex to integrate information [ 112 ]. The authors started from the view that dreaming represents a consciousness experience during sleep. In turn, the level of consciousness corresponds to the amount of integrated information generated by a complex of elements and would only vanish during dreamless sleep or under general anesthesia [ 114 ]. In other words, slow frequencies (1–4 Hz) EEG during sleep are associated with neuronal down-states preventing the emergence of stable causal interactions among cortical areas and provoking the loss of consciousness [ 115 , 116 , 117 ].

Although the application of brain stimulation techniques in the field of sleep and dreaming represents a promising research area [ 118 ], the available evidence is far from being conclusive.

However, the direct manipulation of brain activity could give us powerful insights in understanding the neural substrates of dream recall.

3.3. Insights for Future Research

Dream research using EEG and neuroimaging techniques have advanced significantly over the last twenty years. However, several issues are still opened and unsolved. To direct future studies, we tried to summarize the questions deserving further deepening.

Firstly, we have to underline that, to date, different techniques allowed researchers to assess just some pieces of knowledge on dream experience. We highlighted that some studies aiming to detect structural and functional measures or brain activation patterns revealed exclusively stable/trait-like characteristics of subjects who recall their dreams (e.g., [ 6 , 7 , 15 , 49 ]).

Many researchers agree to consider PSG/EEG studies with provoked awakenings the gold-standard method to study dream recall (DR), since it is a tool easy to apply. On one hand, PSG allows to carry out very different experimental protocols (nap, multiple naps, multiple awakenings, multiple sleep latency test, undisturbed night). On the other hand, these have often led to conflicting or partially consistent findings.

In this respect, we point that most investigations tried to find the possible relations between electrical activity patterns and DR without considering a crucial issue: are these patterns state-like or trait-like phenomena? In this respect, EEG within-subject protocols (e.g., [ 8 , 9 , 13 , 16 ]) would ensure that the observed patterns depend on the specific physiological background and not the stable EEG characteristics of recorded subjects. However, no data about how these aspects impact on dream contents are still available.

Further, very few studies tried to combine different methods to detect the neurobiological correlates of dreaming (e.g., [ 49 ]). We also need to consider that variation in dream experience can be due to homeostatic and circadian factors which can play a role in the specific scenario of physiological activity during sleep [ 119 ]. For instance, Goodenough et al. [ 120 ] found dream reports with higher total word count followed NREM sleep, in the process of transitioning to REM [ 120 ]. To date, just a few studies have investigated the influence of circadian and homeostatic aspects on dreaming [ 4 , 9 , 12 , 16 ] showing, overall, that they do not affect the EEG pattern related to the presence/absence of DR. However, none of these studies collected dream reports, so no information is available about the relation between the EEG variations, due to these processes, and the qualitative/quantitative features of DR.

Multiple awakenings and within-subject studies can assess both state- and trait-like variables and their interactions with homeostatic pressure and chronobiological variables (e.g., ultradian oscillation, a switch-like circadian oscillation, or 28-day cicatrigintan rhythm for women, [ 119 ]). However, the use of the serial awakening method to investigate the EEG correlates of dreaming has been criticized (e.g., [ 121 ]) since this paradigm has not always produced consistent results [ 68 ]. We believe that complementary studies with a focus on undisturbed sleep and neuroimaging studies can help put the pieces of this complex puzzle together. More directly, future studies should be taken into consideration the possibility of integrating different techniques to solve the state-/trait-like issue and assess the role of brain networks involved in dreaming at multiple levels (structural measures, activation patterns, EEG activity in cortical and subcortical regions).

We highlighted that two main models (activation and continuity) may explain the production and retrieval of dream experience. Overall, in line with the earlier neuropsychological findings, more recent neuroimaging and EEG data confirmed that specific brain networks drive many aspects of dream experience. The current literature suggested that the local activation of the posterior zone and medial prefrontal cortex (mPFC) are involved with dream cognition [ 15 , 51 ]. Hence, the neuroscientific discoveries are in line with the continuity hypothesis since they mostly converge to show that the same brain networks responsible for cognitive functions during wakefulness play a role in dream experience. Additionally, specific EEG oscillations underlying memory processing such as theta and alpha bands have proven to predict DR (e.g., [ 5 ]). In parallel, the presence of a significant level of arousal expressed in terms of higher rapid EEG frequency (beta, gamma range) and/or lower slow EEG frequency (e.g., [ 9 , 13 , 16 ]) seem to conflict with the idea of “continuity” between sleep and wakefulness. Notwithstanding, the models based on physiological activation and shared mechanisms across the different state of consciousness are not mutually exclusive: both provide a better comprehension of the neural correlates of a complex phenomenon as dreaming.

It should be speculated that the continuity hypothesis is mostly related to the mechanism involved to recall specific memories (autobiographical and/or emotionally charged memories) inserted to some extent in dream activity. At the same time, the brain activation during sleep would produce a physiological background promoting the storage of oneiric-mnestic traces. In keeping with the idea that the two models are complementary, Koulack and Goodenough [ 61 ] suggested that only salient events or particularly emotional intense contents that stand out compared to other stimuli are processed at the time of arousal and are more likely recalled after awakening.

Here, we have also mentioned that dream contents and the level of consciousness during sleep may be manipulated. The few studies investigating induction of lucid dreams with electrical brain stimulation (tDCS and tACS) have found fascinating but sparse effects on dreaming [ 26 , 107 , 108 , 109 ], and a method for reliably inducing lucid dreams by electrical stimulation of the brain is still yet to be found. Moreover, further applications of TMS should be expanded to change the level of consciousness [ 112 ]. For instance, high-frequency repetitive TMS (rTMS) may be applied to enhance neuronal excitability in focal cortical regions inducing specific EEG frequency to produce lucid dreaming [ 122 ].

In our opinion, the application of brain stimulation to modulate the level of consciousness and induce specific dream contents may give us crucial insights on the neural correlates of dreaming. In this vein, future studies may plan to stimulate different brain areas by using different types of stimulation.

More in general, both results from sensory and brain stimulation are in line with the idea that the neural bases of specific oneiric contents coincide with those related to corresponding waking behavioral and cognitive processes. Undoubtedly, the study of incorporation is still challenging for researchers mostly because of the intrinsic access limitation to dream that cannot be investigated directly.

A novel approach to achieve a better understanding of this phenomenon is represented by the study on parasomnia-like events. Many findings pinpoint that the content of dream enacting behaviors (DEB) had a high level of concordance with subsequent dream reports [ 123 ]. In this respect, several studies agree on the idea that REM Behavior Disorders (RBD) or sleep talking could be considered a viable way to access to dream contents [ 123 , 124 ]. Moreover, some pioneering studies by the group of Isabelle Arnulf focused on sleepwalking and sleep terrors [ 125 , 126 ]. It should be noted that the uniqueness of these studies is represented by the dissociation between cortical areal showing slow-waves and motor cortices showing a wake-like activation [ 127 ]. In such way, this approach may provide useful insights on the dream characteristics associated to SWA and, more in general, to deep sleep.

Hence, we think that the investigation on parasomnia is promising future directions in dream research since the behavioral enactment (e.g., movements, verbalizations, emotional reactions) of dream scenario would allow researchers to look at dream contents while the subject is still asleep directly.

Table 1 illustrates the mentioned neuroscientific techniques, with their strengths and weaknesses, highlighting that each of these approaches led researchers to identify specific aspects of neural correlates of dreaming and solve different dreaming-related issues.

Neuroscience techniques for the study of dreaming.

iEEG, Intracranial-Electroencephalography; PSG, Polysomnography; VPSG, video-polysomnography; EEG, Electroencephalography; hd-EEG, High-Density Electroencephalography; EOG, Electrooculography; EMG, Electromyography; DEB, Dream Enacting Behavior; fMRI; Functional Magnetic Resonance Imaging; PET, Positron-Emission Tomography; MRI; DTI, Diffusion Tensor Imaging; DR, Dream Recall; tES, Transcranial Electrical Stimulation; tDCS, Transcranial Direct Current Stimulation, tACS, Transcranial Alternating Current Stimulation; TMS, Transcranial Magnetic Stimulation, rTMS, Repetitive Transcranial Magnetic Stimulation.

3.4. Conclusions

We report in Table 2 a short synopsis of the studies that have been discussed in this review as a function of the level of investigation (i.e., EEG, iEEG, etc.) and of the support to a continuity or activation hypothesis.

Main findings on neurobiological mechanisms of dreaming.

EEG, Electroencephalography; PSG, Polysomnography; iEEG, Intracranial-Electroencephalography; DR, dream recall; REM, Rapid Eye Movement; NREM, Non-Rapid Eye Movement.

To sum up the points in a future research agenda, we emphasize that the following issues should be included:

• Studies aimed to collect dream reports using within-subjects design protocols, taking under control the homeostatic and circadian variables, should be planned.
• Protocols collecting dream recall (DR) with high-density EEG (high spatial resolution) should be implemented to replicate previous findings on the relationship between specific oscillations and qualitative dream features. Source localization technique should be integrated to maximize the matching between cortical regions and dream contents.
• Combining different techniques would benefit the dream research, also integrating brain stimulation techniques (tDCS, tACS and TMS).
• Providing longitudinal data in clinical and healthy samples to observe potential changes in the brain patterns and dream contents may help face the state-/trait-like issue.
• Studies with a focus on parasomnia-like events would allow overcoming the problem of correspondence between specific time/stage of sleep and the DR generation.

Finally, it should be considered that collaboration among the scientists in the relatively wide dream community could reduce the high fragmentation of the current literature, providing, for instance, a more unified method to study dream experience.

Author Contributions

Conceptualization, S.S. and L.D.G.; methodology, S.S. and V.A.; software, S.S. and V.A. validation, S.S., V.A., M.G., A.M.G., and L.D.G., investigation, S.S. and V.A.; resources, S.S. and V.A.; writing—original draft preparation, S.S. and V.A.; writing—review and editing, M.G., A.M.G. and L.D.G.; visualization, S.S., V.A. and M.G.; supervision, A.M.G. and L.D.G. All authors have read and agreed to the published version of the manuscript.

This research received no external funding.

Conflicts of Interest

The authors declare no conflict of interest.

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Mood pp 159–196 Cite as

The Continuity Hypothesis

• William N. Morris 2  

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Part of the book series: Springer Series in Social Psychology ((SSSOC))

The idea that everyday good and bad moods might differ from clinically significant depression and mania only in terms of degree rather than kind suggests that these states are on continua such that intensification of normal processes can produce disorder. This view, sometimes referred to as the continuity hypothesis (e.g., see Beck, 1967; Blatt et al., 1976; Coyne, 1986; Depue et al., 1981; Eastwood et al., 1985; Gotlib, 1984), is a controversial one that has tended to divide clinicians and researchers into two camps. Opposed to the continuity hypothesis are those who take a biomedical orientation toward affective disorder, regarding it as a disease traceable to abnormal neurophysiological and biochemical processes (e.g., Whybrow & Mendels, 1969; Siever & Davis, 1985). In support of the hypothesis, one finds an uneasy alliance of behaviorally, cognitively, and psychoanalytically oriented psychologists and psychiatrists who believe that psychological processes and vulnerabilities are sufficient to explain clinically significant depression. At stake are the implications of the continuity hypothesis: that we can learn more about the nature of affective disorder through “analog” research, that is, by studying the moods of “normal” individuals, and that psychological therapies can be effective remedies, especially to the extent that they are based on evidence about ways to successfully eliminate everyday bad moods.

• Bipolar Disorder
• Positive Affect
• Affective Disorder
• Depressive Episode
• Manic Episode

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Morris, W.N. (1989). The Continuity Hypothesis. In: Mood. Springer Series in Social Psychology. Springer, New York, NY. https://doi.org/10.1007/978-1-4612-3648-1_7

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