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Dreams may be crucial in mammalian memory processing. Important information acquired while awake may be reprocessed during sleep TheMeaning ofDreams By Jonathan Winson Throughout history, human beings have sought to understand the meaning of dreams. The ancient Egyptians believed dreams possessed oracular pow- state, a period of several minutes when thoughts consist of frag- er—in the Bible, for example, Joseph’s elucidation of Pharaoh’s mented images or minidramas. The hypnogogic state is followed dream averted seven years of famine. Other cultures have in- by slow-wave sleep, so called because at that time the brain terpreted dreams as inspirational, curative or alternative reality. waves of the neocortex (the convoluted outer mantle of the During the past century, scientists have offered conflicting psy- brain) are low in frequency and large in amplitude. These sig- chological and neuroscientific explanations for dreams. In nals are measured as electroencephalographic (EEG) recordings. 1900, with the publication of The Interpretation of Dreams, Researchers also discovered that a night’s sleep is punctu- Sigmund Freud proposed that dreams were the “royal road” to ated by periods in which the EEG readings are irregular in fre- the unconscious, that they revealed in disguised form the deep- quency and low in amplitude—similar to those observed in est elements of an individual’s inner life. awake individuals. These periods of mental activity are called More recently, in contrast, dreams have been characterized REM sleep. Dreaming takes place solely during these periods. as meaningless, the result of random nerve cell activity. Dream- While in REM sleep, motor neurons are inhibited, preventing ing has also been viewed as the means by which the brain rids the body from moving freely but allowing extremities to remain itself of unnecessary information—a process of “reverse learn- slightly active. Eyes move rapidly in unison under closed lids, ing,” or unlearning. breathing becomes irregular, and heart rate increases. Based on recent findings in my own and other neuroscien- The first REM stage of the night follows 90 minutes of slow- tific laboratories, I propose that dreams are indeed meaningful. wave sleep and lasts for 10 minutes. The second and third REM Studies of the hippocampus (a brain structure crucial to mem- periods follow shorter slow-wave sleep episodes but grow pro- ory), of rapid eye movement (REM) sleep and of a brain wave gressively longer themselves. The fourth and final REM inter- called theta rhythm suggest that dreaming reflects a pivotal as- val lasts 20 to 30 minutes and is followed by awakening. If a pect of the processing of memory. In particular, studies of theta dream is remembered at all, it is most often the one that oc- rhythm in subprimate animals have provided an evolutionary curred in this last phase of REM sleep. clue to the meaning of dreams. They appear to be the nightly This sleep cycle—alternating slow-wave and REM sleep— record of a basic mammalian memory process: the means by appears to be present in all placental and marsupial mammals. which animals form strategies for survival and evaluate current Mammals exhibit the various REM-associated characteristics experience in light of those strategies. The existence of this pro- observed in humans, including EEG readings similar to those cess may explain the meaning of dreams in human beings. of the awake state. Animals also dream. By destroying neurons in the brain stem that inhibit movement during sleep, research- Stages of Sleep and Dreaming ers found that sleeping cats rose up and attacked or were star- THE PHYSIOLOGY OF DREAMING was first understood in tled by invisible objects—ostensibly images from dreams. 1953, when researchers characterized the human sleep cycle. By studying nonprimate animals, scientists have discovered They found that sleep in humans is initiated by the hypnogogic additional neurophysiological aspects of REM sleep. They de- 54 SCIENTIFIC AMERICAN Updated from the November 1990 issue COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC. JACOB’S LADDER, painted in 1973 by Marc Chagall, depicts a biblical story. When Freud wrote The Interpretation of Dreams, the phys- Jacob dreams of angels ascending to and descending from heaven on a ladder. iology of sleep was unknown. In light of the discovery of REM sleep, certain elements of his psychoanalytic theory were mod- termined that neural control of this stage of the sleep cycle is cen- ified, and the stage was set for more neurologically based the- tered in the brain stem (the brain region closest to the spinal ories. Dreaming came to be understood as part of a biologically cord) and that during REM sleep neural signals—called pontine- determined sleep cycle. Yet the central concept of Freud’s theo- geniculate-occipital (PGO) cortex spikes—proceed from the ry—namely, the belief that dreams reveal a censored representa- brain stem to the center of visual processing, the visual cortex. tion of our innermost unconscious feelings and concerns—con- Brain stem neurons also initiate a sinusoidal wave (one resem- tinues to be used in psychoanalysis. bling a sine curve) in the hippocampus. This brain signal is called Some theorists abandoned Freud altogether following the theta rhythm. neurological discoveries. In 1977 J. Allan Hobson and Robert At least one animal experiences slow-wave but not REM McCarley of Harvard Medical School proposed the “activa- sleep—and, consequently, does not exhibit theta rhythm when tion-synthesis” hypothesis. They suggested that dreaming con- asleep. This animal is the echidna, or spiny anteater, an egg-lay- sists of associations and memories elicited from the forebrain ing mammal (called a monotreme) that provides some insight (the neocortex and associated structures) in response to ran- into the origin of dreaming. The absence of REM sleep in the dom signals from the brain stem such as PGO spikes. Dreams echidna suggests that this stage of the sleep cycle evolved some were merely the “best fit” the forebrain could provide to this 140 million years ago, when marsupials and placentals diverged random bombardment from the brain stem. Although dreams from the monotreme line. (Monotremes were the first mammals might at times appear to have psychological content, their to develop from reptiles.) bizarreness was inherently meaningless. By all evolutionary criteria, the perpetuation of a complex The sense, or plot, of dreams resulted from order that was brain process such as REM sleep indicates that it serves an im- imposed on the chaos of neural signals, Hobson said. “That or- portant function for the survival of mammalian species. Un- der is a function of our own personal view of the world, our re- SCALA/ART RESOURCE, ©2002 ARTISTS RIGHTS SOCIETY (ARS), NEW YORK/ADAGP, PARIS derstanding that function might reveal the meaning of dreams. mote memories,” he wrote. In other words, the individual’s www.sciam.com THE HIDDEN MIND 55 COPYRIGHT 2002 SCIENTIFIC AMERICAN, INC. emotional vocabulary could be relevant ment by PGO waves and their own were a running record of these parasitic to dreams. In a further revision of the knowledge of the behavior of stimulated thoughts—material to be purged from original hypothesis, Hobson also sug- neural networks, Crick and Mitchison memory. “We dream to forget,” Crick gested that brain stem activation may postulated that a complex associational and Mitchison wrote. serve merely to switch from one dream neural network such as the neocortex The two researchers proposed a revi- episode to another. might become overloaded by vast amounts sion in 1986. Erasure of parasitic thoughts of incoming information. The neocortex accounted only for bizarre dream con- Reverse Learning could then develop false, or “parasitic,” tent. Nothing could be said about dream ALTHOUGH HOBSON and McCarley thoughts that would jeopardize the true narrative. Furthermore, dreaming to for- had presented an explanation of dream and orderly storage of memory. get, they said, was better expressed as content, the basic function of REM sleep According to their hypothesis, REM dreaming to reduce fantasy or obsession. remained unknown. In 1983 Francis sleep served to erase these spurious asso- None of these hypotheses seems to Crick of the Salk Institute for Biological ciations on a regular basis. Random PGO explain adequately the function of dream- Studies in San Diego and Graeme Mitchi- waves impinged on the neocortex, re- ing. On the one hand, Freud’s theory son of the University of Cambridge pro- sulting in erasure, or unlearning, of the lacked physiological evidence. (Although posed the idea of reverse learning. Work- false information. This process served an Freud had originally intended to describe ing from the Hobson-McCarley assump- essential function: it allowed the orderly the neurology of the unconscious and of tion of random neocortical bombard- processing of memory. In humans, dreams dreams in his proposed “Project for a Sci- entific Psychology,” the undertaking was premature, and he limited himself to psy- Prefrontal cortex choanalysis.) On the other hand, despite revisions to incorporate elements of psy- Septum chology, most of the later theories denied that dreams had meaning. Exploring the neuroscientific aspects of REM sleep and of memory processing seemed to me to hold the greatest poten- tial for understanding the meaning and function of dreams. The key to this re- search was theta rhythm. Theta rhythm was discovered in 1954 in awake animals by John D. Green and Arnaldo A. Arduini of the Universi- ty of California at Los Angeles. The re- searchers observed a regular sinusoidal signal of six cycles per second in the hippocampus of rabbits when the animals were apprehensive of stimuli in their en- vironment. They named the signal theta Visual rhythm after a previously discovered HIPPOCAMPUS Entorhinal cortex EEG component of the same frequency. cortex Theta rhythm was subsequently re- Hippocampus corded in the tree shrew, mole, rat and Brain stem cat.

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