Dreaming is a mental state associated with sleep whose characteristics have important implications for theories of normal consciousness as well as for its extreme derangement in major mental illness. The advent of modern sleep laboratory science has provided an objective aspect to the study of dreams and has opened the way to a brain-based theory of dreaming. Recent excitement about the prospects of this agenda stems from the application of brain imaging techniques that allow dream activity in humans to be correlated with the selective activation and deactivation of various brain regions. The modern scientific study of dreaming thus provides an avenue of access to the mind-body problem, one of the most obdurate philosophic conundrums of human history.
While dreaming can be broadly defined as any kind of mental activity occurring in sleep, most people and most brain scientists are interested in a more specific state of mind that is normally unique to sleep. When we say ‘I had the craziest dream last night’ we refer to a conscious experience during sleep marked by visual imagery, delusional misidentification of our state as waking, difficulties with thought processes, emotional intensification, and very significant recent memory loss. It is important to emphasize this particular definition of dreaming for two reasons. The first reason is that this kind of dreaming is so highly correlated with the physiology of the stage of sleep known as paradoxical or REM (rapid eye movement) sleep as to invite an integration of dream psychology with the specific brain processes of REM. The second is that this kind of dreaming shows many of the formal features of such major dysfunctional states as schizophrenia, manic depression, and organic psychoses. But even these intense dreams are not restricted to REM sleep. Furthermore, many other cataclysmic states of mind, like the night terrors of normal children and the horrifying replay of experiences in people who have been brutalized or traumatized, occur almost exclusively in other, non-REM stages of sleep (NREM).
Until recently, the traditional approach to understanding dreaming has emphasized its narrative or scenario-like character and attempted to elaborate an interpretive scheme that could bring order to the emotional and cognitive chaos of dreaming. The most famous approach of this type is the psychoanalytic theory of Sigmund Freud, who abandoned his early hopes for a brain-based approach because the necessary neurophysiological data were then non-existent. Freud was therefore obliged to account for all of the formal properties of dreaming in psychological terms, a heavy burden, which caused his ingenious speculations to become Byzantine in their complexity and comical in their interpretive oversimplification.
For Freud, every dream was caused by unconscious wishes that were released in sleep. These forbidden impulses threatened to invade consciousness and to cause an awakening if their unacceptable meaning was not censored and disguised by symbolic transformation. The hallucinatory character of dreams, the bizarreness of dream perception, the defective cognition, and even the memory loss were all ascribed to psychological defence mechanisms whose function was to protect sleep. For Freud, the common existence of strong negative emotion in dreams was a factor that his theory could not explain.
The discovery of the biphasic cycle of NREM and REM sleep and the possibility of waking people up at particular stages of sleep in the laboratory, immediately provided an alternative approach. The flood of data quickly overthrew many age-old assumptions about dreaming, as well as challenging Freud's ideas. For example, it was soon clear that everyone dreams, every night, and that dreaming of the vivid kind emphasized here occupies finite amounts of time — up to one and a half or even two hours of sleep. Unless we awaken spontaneously, or are awakened by an external stimulus (such as an experimenter), we may recall nothing. More interestingly, the most vivid and sustained dreaming occurs in association with REM sleep. Moreover, dreaming intensifies the intermittent bursts of eye movement that punctuate that sleep phase. These discoveries showed that the subjective experience had been not only grossly inadequate but also downright misleading in suggesting, for example, that dreams are rare, colourless, evanescent events that occur in the instant before awakening. Furthermore, Freud's speculative hypotheses about the instigation of dreams by unconscious wishes, and about censorship and repression moulding the content of dreams, were called into question.
To develop the detailed brain-based theory of dreaming that Freud himself yearned for, it was also important to utilize information from animal experiments that identified the specific brain cells and molecules involved in REM sleep generation in all mammals, including man. For example, the electrical activity of the brain (recorded by electroencephalography) and the movements of the eyes are very similar in the awake state and during REM sleep. However, in REM, the activated brain is operating without those neurotransmitter chemicals known to be necessary to order attention, perception, thought, and memory. No wonder dreaming is so chaotic, bizarre, unfocused, and unremembered. No need to postulate psychological defences, symbolic transformations, or any other Freudian fantasy to understand what is going on.
This is not to say that dream narratives are without interest or without meaning. The interpretive stance taken by modern dream science is that the emotionally salient aspects of dreams mean exactly what they appear to mean, while the meaningless nonsense is simply a by-product of the brain's physiological handicap. In other words, dream psychology is of interest precisely because it so directly and undefensively reveals the dreamer's psychological concerns. On this view, dream meaning is transparent, not disguised, and is synthesized as the dreamer tries to make sense of the activation of his emotional and perceptual brain without the aid of the organizing and directive powers available to it during waking.
Neuropsychological evidence
The capacity to deduce regional activation patterns in the human brain during natural sleep is afforded by imaging techniques, such as positron emission tomography (PET) and functional magnetic resonance imaging (fMRI), which measure differences in local blood flow and thus infer the increases in nerve cell activity that call for more oxygen. For the first time in history scientists are able to study the depths of the human brain in action and to correlate the differences between, say, waking and REM sleep with the psychology typical of these states.
Recent PET data affirm the importance of the brain stem in REM sleep dream generation, that was suggested by animal studies, but they also emphasize the importance of emotion centres in the limbic system. The integration of the emotions and internally-generated perceptions of dreams is facilitated by selective activation of a specific cortical zone at the junction of the parietal, temporal, and occipital lobes, whose damage typically results in disintegration of these functions, and to a loss of dreaming. Dreaming is also lost when strokes damage the deep basal forebrain area thought to mediate emotional and motivational drive, a finding that would have thrilled Sigmund Freud in 1895 when he was still struggling to produce a brain-based theory of the mind.
But the most significant finding of all may be the discovery that the brain region responsible for working memory, critical evaluation of behavioural options, and decision-making is selectively deactivated in REM sleep. How, and why, this critical area of the frontal cortex escapes the activation process that is otherwise at least as prominent in REM as in waking is as yet unknown, but it is irresistibly attractive to speculate that it may be related to the regional differences in chemical modulation of the brain that have been discovered in animal studies.
Whatever the answer to detailed questions such as these, it can safely be concluded that no one interested in dreaming, including the most die-hard psychoanalyst, can any longer afford to be either ignorant of or indifferent to the findings of modern brain research.
— J. Allan Hobson
See also brain stem; consciousness; emotion, biological basis of; limbic system; mind-body problem; sleep; sleep disorders.
