REM sleep and dreaming
The history of dreaming long predates its scientific investigation. Aserinsky and Kleitman15 showed for the first time that dreaming was associated with a state of low voltage EEG and eye movements during sleep that they called REM sleep. This discovery was followed by the surprising discovery that the physiological state that had been linked to dreams in humans could be identified in most mammals, suggesting that animals dream. The state also recurred in a regular rhythm, with a cycle duration that was positively correlated with body and brain mass16.
The assumption that animals have dream-like experience as we do, has to be tempered by more recent findings that human dreaming can be abolished by relatively small cortical lesions that do not interfere with other aspects of REM sleep17. Furthermore, it has been established that some humans do not report dreaming. This finding has typically been dismissed as being due to the rapid forgetting of dreams that most of us experience. If a person awakens a few minutes after the end of a REM sleep period they will not report dreaming. But in an impressive test of this hypothesis, non-dream reporters were awakened from REM sleep at all times of the night and many still reported no dreams18. Furthermore, human infants have huge amounts of REM sleep, yet it is inconceivable that they could have dream experiences that are similar to adult dreams. Children under the age of 5-6 have dream recall from only 20-30% of REM awakenings, the reported dreams are brief, fragmentary and static. Recall is still below 30% at 8 years of age, not reaching the adult level of 80% until ages 9-1119. Furthermore, the primitive mammal platypus, the animal with the greatest amount of REM sleep, does not have cortical activation during most of this state20, again casting doubt on the often assumed linkage between REM and higher cognitive processes including dreaming.
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Palagini and Rosenlicht 12 review the early art and literature linking dreaming to divine and prophetic messages. The Greek Artemidorus Daldianus catalogued over 30,000 different types of dreams emphasizing their prophetic meaning. In Greek mythology, Hypnos, the god of sleep, Oneiros the god of dreams and Morpheus, the dream shaper, worked together to reduce human suffering. Plato proposed that repressed bestial desires can be expressed in dreams, presaging Freudian 19th and 20th century speculations. Aristotle rejected the idea that dreams are prophetic, indicating that they were the result of sensations occurring in the absence of perception. Medieval concepts applied the Hippocratic theory of humors (blood, black bile, yellow bile and phlegm) to explain dreams, with each fluid causing “vapors” that produced particular types of dreams. Freud, Jung and others used dreams to understand and treat emotional problems on the assumption that dreaming is a meaningful reflection of unconscious mental functioning.
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Controversies over dream meaning continue into the modern era. The idea that the movements of the eyes scan the dream images have been disputed21, but receives support in studies of REM sleep behavior disorder22. It has been proposed that dreams are essentially a passive response to stochastic ascending input from the brainstem REM sleep generator23. However, systematic analyses of dreams show them for the most part to be structured with regular developmental characteristics simulating waking experience. This data indicates that, bizarre dream characteristics are relatively rare in contrast to what one might expect from pulsatile brainstem activation. Daily events are not typically integrated into dreams but clearly waking concerns and interests shape dreams as they do waking cognition24.
Although all brain regions are capable of plasticity a few brain regions including the olfactory bulb and the dentate gyrus of the hippocampus actually generate new neurons throughout the lifespan, likely compensating the relatively rapid loss of old cells in these same structures. Sleep deprivation, particularly REM sleep deprivation, interferes with the neurogenesis either by preventing an essential step in the process of neurogenesis that occurs only during sleep, or by disrupting this process by the stress elicited by the deprivation procedure25. However, as pointed out above, total blockade of REM sleep does not interfere with learning4.
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