BEHAVIORAL AND BRAIN SCIENCES (2000) 23, 793–1121 Printed in the United States of America The reinterpretation of dreams: An evolutionary hypothesis of the function of dreaming Antti Revonsuo Department of Philosophy, Centre for Cognitive Neuroscience, University of Turku, Turku FIN-20014, Finland [email protected] www.utu.fi/research/ccn/consciousness.html Abstract: Several theories claim that dreaming is a random by-product of REM sleep physiology and that it does not serve any natural function. Phenomenal dream content, however, is not as disorganized as such views imply. The form and content of dreams is not ran- dom but organized and selective: during dreaming, the brain constructs a complex model of the world in which certain types of elements, when compared to waking life, are underrepresented whereas others are over represented. Furthermore, dream content is consistently and powerfully modulated by certain types of waking experiences. On the basis of this evidence, I put forward the hypothesis that the biological function of dreaming is to simulate threatening events, and to rehearse threat perception and threat avoidance. To evaluate this hypothesis, we need to consider the original evolutionary context of dreaming and the possible traces it has left in the dream con- tent of the present human population. In the ancestral environment human life was short and full of threats. Any behavioral advantage in dealing with highly dangerous events would have increased the probability of reproductive success. A dream-production mechanism that tends to select threatening waking events and simulate them over and over again in various combinations would have been valuable for the development and maintenance of threat-avoidance skills. Empirical evidence from normative dream content, children’s dreams, recurrent dreams, nightmares, post traumatic dreams, and the dreams of hunter-gatherers indicates that our dream-production mecha- nisms are in fact specialized in the simulation of threatening events, and thus provides support to the threat simulation hypothesis of the function of dreaming. Keywords: dream content; dream function; evolution of consciousness; evolutionary psychology; fear; implicit learning; nightmares; re- hearsal; REM; sleep; threat perception Introduction get article leads to the slightly surprising conclusion that dreaming does have a well-defined and clearly manifested Dreaming is a universal feature of human experience, but biological function after all. In section 1, I clarify the nature there is no convincing explanation as to why we should ex- of the basic question: What exactly is it that we want to un- perience dreams during sleep. Why do we have vivid, in- derstand when we inquire about the function of dreaming? tense, and eventful experiences while we are completely The answer is that we need a clear idea of both what the unaware of the world that physically surrounds us? Couldn’t phenomenon of dreaming is and of the sense in which we we just as well pass the night completely nonconscious? The are using the word “function.” In section 2, we review the function of dreaming seems to be a persistent mystery, al- currently dominant views on the function of dreaming in though numerous suggestions have been put forward about the possible functions it might serve. The leading neu- rocognitive theories, however, seem to have given up the Antti Revonsuo is a Fellow of the Academy of Fin- hope of identifying any useful function for dreaming at all. land at the University of Turku. He has published They cannot provide us with an answer to the question widely in cognitive neuroscience, neuropsychology, and “Why do we dream?” Instead, they seem to imply that we consciousness studies. His research aims at under- dream for no particular reason at all: Dreaming is biologi- standing consciousness as a natural biological phenom- cally epiphenomenal. Dream consciousness is viewed as enon and at fruitful interaction between philosophical some sort of random noise generated by the sleeping brain and empirical research in the study of consciousness. as it fulfills various neurophysiological functions during He is co-editor of two books on consciousness, Con- sciousness in philosophy and cognitive neuroscience REM (rapid eye movement) sleep. (Erlbaum, 1994) and Beyond dissociations: Interaction Although the prospects for discovering useful functions between dissociated implicit and explicit processing for dreaming look rather bleak, the empirical evidence (Benjamins, 2000). Revonsuo is the European Editor of should be reevaluated once more from a truly multidisci- Consciousness and Cognition, and currently on the plinary point of view, including dream content analysis, the board of Directors of the Association for the Scientific neurophysiology of dream sleep, and evolutionary psychol- Study of Consciouness. ogy. The exploration that I undertake in the present tar- © 2000 Cambridge University Press 0140-525X/00 $12.50 877 Revonsuo: Reinterpretation of dreams the cognitive and neuroscientific literature as well as in the threat-simulation mechanism. In the final section, the theory more clinically oriented dream psychology. The most com- is compared with neurocognitive theories of dreaming. mon view in cognitive neuroscience is that dreaming has no Taken together, this target article aims to show that the function whatsoever. In clinical literature, the function of threat-simulation theory of dreaming integrates a consider- dreaming has been linked with problem solving and psy- able body of data from multiple sources in a theoretically chological adaptation, but the direct empirical evidence meaningful way. The theory treats the conscious phenom- bearing on such functions remains scarce. In section 3 we enal experience of dreaming as a natural biological phe- point out that none of the previous theories have placed nomenon best understood from the combined viewpoints dreaming in the appropriate context for evaluating its pos- of psychology, evolutionary biology, and cognitive neuro- sible biological functions: the human ancestral environ- science. This multidisciplinary treatment, I hope, manages ment in which the dreaming brain was evolving for hun- to clarify the mystery of why we dream. dreds of thousands of years. If dreaming does have any biologically adaptive functions, they must have been effec- tive in the evolutionary context, if anywhere. 1. What is it that we want to understand when we In the rest of the article I argue that switching the inquire about the function of dreaming? context in such a way puts dreaming into an entirely new light, which suggests that the biologically adaptive func- We should first make clear what it is we are asking when we tion of dreaming is to simulate threatening events in order inquire about the function of dreaming. We must explicate to rehearse threat perception and the appropriate threat- what we mean by dreaming and what we mean by function. avoidance skills and behavioral programs. I emphasize that to claim threat simulation as the biological function 1.1. What is dreaming? of dreaming is not to claim that every single dream of every single individual should realize this function. It is Dreaming refers to the subjective conscious experiences only to claim that in certain adaptively important situa- we have during sleep. We may define a dream as a subjec- tions with certain ecologically valid cues, the system does tive experience during sleep, consisting of complex and or- become fully activated, and this is the principal reason why ganized images that show temporal progression (Farthing dreaming was selected for during our evolutionary history. 1992). Questions regarding the function of dreaming must The threat simulation theory of dreaming is expressed be clearly distinguished from those regarding the function here in the form of six propositions, each of which is em- of REM sleep. Dreaming is a subjective conscious experi- pirically testable. The propositions can be summarized as ence, while REM sleep is a physiologically defined stage of follows: sleep. Furthermore, as is now clear, REM sleep is neither a 1. Dream consciousness is an organized and selective necessary nor a sufficient physiological condition for dream- simulation of the perceptual world. ing, although it seems to be the typical and perhaps optimal 2. Dream consciousness is specialized in the simulation physiological condition in which fully realized dreams are of threatening events. brought about (Pivik 1991). As Foulkes and Cavallero (1993, 3. Nothing but exposure to real threatening events fully p. 9) emphasize, dreaming needs a level of explanation in- activates the threat-simulation system. dependent of the neurophysiological level at which REM 4. The threat simulations produced by the fully activated sleep is defined, because “there almost certainly is REM system are perceptually and behaviorally realistic re- sleep without dreaming and . there certainly is dreaming hearsals of threatening events. without REM sleep. No account of the distinctive physiol- 5. The realistic rehearsal of these skills can lead to en- ogy of REM sleep could provide either a necessary or a suf- hanced performance regardless of whether or not the train- ficient explanation of dreaming.” Thus, the question we will ing episodes are explicitly remembered. be exploring is: Does it serve any useful function to have, 6. The ancestral environment in which the human brain during sleep, the sorts of conscious subjective experiences evolved