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Abstracts from Science issue on dreaming 2001

Sweet Dreams Are Made of This

Peter Stern

Dreams have always fascinated humans, and throughout history there have been numerous hypotheses and speculations concerning their meaning and function. However, only in the second half of the 20th century, after the introduction of electroencephalographic recordings and the discovery of rapid eye movement (REM) sleep, have these phenomena been the subject of serious scientific investigation. It is amazing that we can make incredibly accurate predictions about the movement or the composition of stars in galaxies millions of light years away while, much much closer to home, many crucial processes in our own heads remain shrouded in mystery. Every human being experiences the phenomena of sleep and dreams, and most people take them for granted without ever questioning what their biological purpose might be. There have always been teleological arguments for why we need to sleep, but they can mostly be dismissed as descriptive without any real explanatory value. Indeed, why do homeothermic animals need to rest and sleep at all, which makes them particularly vulnerable to predators? Wouldn't it be better and more efficient if they were up and running 24 hours a day? So it seems even more mysterious that during sleep we also experience periods of dreaming, with their sometimes bizarre, incoherent, and highly unpredictable contents.

A large body of literature about dreaming has been assembled over recent years. In this special issue of Science, we have tried to take stock and review the present state of our knowledge concerning sleep and dreams and their relation to memory, and we have also tried to indicate the blank spaces on our map. Maquet (p. 1048) reviews the recent literature on the role of sleep for memory formation and puts forward a testable hypothesis concerning the task of experience-dependent reactivation of neuronal populations. Stickgold et al. (p. 1052) deal with the function of sleep, particularly REM sleep, and dreaming for the consolidation of learning and memory tasks. They outline the requirements that should be expected for a comprehensive theory of dreaming. And finally, Siegel (p. 1058) critically reviews the available literature and questions the REM sleep-memory consolidation hypothesis. There is still a lot of conflicting evidence, as well as methodological difficulties, unproven assumptions, and neglected alternative interpretations. By highlighting the holes in the existing theories, we hope this review will be a challenge for scientists working in this field and will stimulate them to improve their experiments and reassess some assumptions and hypotheses they may have taken for granted. To sleep: perchance to query.

The Role of Sleep in Learning and Memory

Pierre Maquet

Sleep has been implicated in the plastic cerebral changes that underlie learning and memory. Indications that sleep participates in the consolidation of fresh memory traces come from a wide range of experimental observations. At the network level, reactivations during sleep of neuronal assemblies recently challenged by new environmental circumstances have been reported in different experimental designs. These neuronal assemblies are proposed to be involved in the processing of memory traces during sleep. However, despite this rapidly growing body of experimental data, evidence for the influence of sleep discharge patterns on memory traces remains fragmentary. The underlying role of sleep in learning and memory has yet to be precisely characterized.

Wellcome Department of Cognitive Neurology, University College London, London WC1N 3BG, UK and Cyclotron Research Centre, University of Liège, Liège 4000, Belgium.

Sleep, Learning, and Dreams: Off-line Memory Reprocessing

R. Stickgold,1* J. A. Hobson,1 R. Fosse,12 M. Fosse1

Converging evidence and new research methodologies from across the neurosciences permit the neuroscientific study of the role of sleep in off-line memory reprocessing, as well as the nature and function of dreaming. Evidence supports a role for sleep in the consolidation of an array of learning and memory tasks. In addition, new methodologies allow the experimental manipulation of dream content at sleep onset, permitting an objective and scientific study of this dream formation and a renewed search for the possible functions of dreaming and the biological processes subserving it.

1 Laboratory of Neurophysiology and Department of Psychiatry, Harvard Medical School, Boston, MA 02115, USA. 2 Institute of Psychology, University of Oslo, Box 1094 Blindem, N-0317 Oslo, Norway. * To whom correspondences should be addressed. E-mail:

The REM Sleep-Memory Consolidation Hypothesis

Jerome M. Siegel

It has been hypothesized that REM (rapid eye movement) sleep has an important role in memory consolidation. The evidence for this hypothesis is reviewed and found to be weak and contradictory. Animal studies correlating changes in REM sleep parameters with learning have produced inconsistent results and are confounded by stress effects. Humans with pharmacological and brain lesion-induced suppression of REM sleep do not show memory deficits, and other human sleep-learning studies have not produced consistent results. The time spent in REM sleep is not correlated with learning ability across humans, nor is there a positive relation between REM sleep time or intensity and encephalization across species. Although sleep is clearly important for optimum acquisition and performance of learned tasks, a major role in memory consolidation is unproven.

Center for Sleep Research, Department of Veterans Affairs, Greater Los Angeles Healthcare System (VA GLAHCS), North Hills, CA 91343, USA, and Department of Psychiatry and Brain Research Institute, University of California, Los Angeles, CA 90024, USA. E-mail:,