Dreams may reflect a memory-processing mechanism inherited from lower species
There are more things in heaven and earth, Horatio, Than are dreamt of in your philosophy--William Shakespeare
Human beings have long sought to understand the meaning or value of dreams. Ancient Egyptians were convinced that dreams possessed oracular powers. In other cultures dreams have been described as inspirational, curative or as alternative reality. Sigmund Freud suggested in his publication The Interpretation of Dreams in 1900 that dreams were “the royal road” to the unconscious, that they revealed in disguised form the deepest elements of a person’s inner life.
Since Freud, scientists have variously suggested that dreams were either totally meaningless—simply the result of random nerve cell activity—or that they were a way for the brain to rid itself of unnecessary information, like ‘unlearning’.
Recent work by scientists, including Jonathan Winson out of Rockefeller University, suggest that Rapid Eye Movement (REM) sleep and brain waves called theta rhythm play key roles in the processing of memory. It is only during REM sleep that we dream. During REM sleep neural signals, called pontine-genidulate-occipital (PGO) cortex spikes, move from the brain stem to the centre of visual processing, the visual cortex. At the same time a sinusoidal wave is initiated in the hippocampus, called theta rhythm.
Discovered in 1954 by John D. Green and Arnaldo A. Arduini of the University of California at Los Angeles, theta rhythm was found in most animals tested, and mostly in awake animals. Researchers found that theta rhythm was evoked during moments when an animal was behaving in ways most crucial to its survival; in other words, it appeared during moments when they were responding to changing environmental information, rather than to something they were genetically encoded for, like feeding or sexual behaviour. The presence of theta rhythm during REM sleep, generated in the hippocampus—where memory processing occurs—suggested that theta rhythm reflected a neural process in which information essential to survival of a species, gathered during the day, was reprocessed into memory during REM sleep.
The hippocampus, together with the neocortex, is believed to provide the neural basis for memory storage, according to Winson. His studies showed that theta rhythm was produced in two regions in the hippocampus: the dentate gyrus and the CA 1 neurons and that the rhythms of these two were synchronous. James N. Ranck, Jr. and Susan Mitchell of the State University of New York identified a third synchronous generator in the entorhinal cortex.
Robert Verdes of Wayne State University then discovered that neurons of the brain stem transmit signals to the septum of the forebrain to activate theta rhythm in the hippocampus and the entorhinal cortex.
Scientists in the 1970s demonstrated that a change in neural behaviour reflecting previous activity, called long-term potentiation (LTP) depended on the presence and phase of theta rhythm. For instance, as a rat explores, brain stem neurons activate theta rhythm.
Not all animals experience REM sleep; the echidna (a monotreme) experiences slow-wave but not REM sleep. As a consequence, it does not exhibit theta rhythm when asleep either. Without theta rhythm during REM sleep, the echidna must rely on its larger prefrontal cortex to perform a dual function: to react to incoming information and to evaluate based on past experience and store new information. Winsom suggests that REM sleep—and dreams—may have evolved to help an animal to survive, by helping to reprocess information.
It is interesting to note that infants and children spend a large amount of their sleep time in REM sleep. Newborns spend eight hours a day in REM sleep; by the time they’re two years old, children have reduced their REM sleep to three hours a day with adults spending about two hours in REM. Scientists suggest that REM sleep stimulates nerve growth.
Winson, Jonathan. 1985. Brain and Psyche: The Biology of the Unconscious. Anchor Press, Doubleday.