Everyone dreams, but some people never recall dreaming. Others remember only a little about a dream they had just before awakening and nothing about earlier dreams. No one recalls every dream and, in general, dreams are very easily forgotten.
People see in most dreams, and they may also hear, smell, touch, and taste in them. Most dreams occur in color, though the color is often recalled only vaguely. Dreaming thought seems to put things together in new and unexpected ways. In some cases, this has led to important scientific discoveries or highly imaginative creative works.
During REM sleep, the pathways that carry nerve impulses from the brain to the muscles are blocked. Therefore, the body cannot move during dreams. Also, the cerebral cortex-the part of the brain involved in higher mental functions-is much more active during REM sleep than during nondreaming sleep. The cortex is stimulated by neurons (nerve cells) that carry impulses from the part of the brain called the brain stem.
Many experts who study dreams also feel that they are related to deep wishes and fears of the dreamer, and several theories explaining the meaning of dreams have been developed. During the 1890's, Sigmund Freud, an Austrian physician who originated psychoanalysis, developed one of the best-known theories of dream interpretation. Freud suggested that dreams are fulfillments of wishes, usually in disguised form. The disguise-or 'dream language'-involves condensation (combining several ideas into one image), displacement (shifting a feeling from one idea or person to another), and symbolism (the use of symbols to represent what cannot be pictured directly).
Some scientists have suggested that biological discoveries about dreaming have made psychological theories of dreaming, such as Freud's, unnecessary or false. These scientists argue that a dream is a meaningless response of the cerebral cortex to random stimulation from the brain stem. However, waking thought is also a response of the cerebral cortex to stimulation, often random, from the brain stem. Therefore, the biology involved does not make dreams meaningless any more than it makes waking thought meaningless. Most psychiatrists and psychologists still consider dreams psychologically meaningful.