The search for the meaning of dreams dates at least to the biblical story of Joseph and The Iliad of Homer. The Book of Genesis quotes Joseph as saying, "The dreams of Pharaoh are one and the same. God has revealed to Pharaoh what he is about to do." The ancient Greek poet describes how Jove sent a lying dream to King Agamemnon, telling it to "say to him word to word as I now bid you." This view of dreams as messages from the deities held sway for at least 4,000 years.
Sigmund Freud's The Interpretation of Dreams, published in 1899, upended this assumption. For Freud, the father of psychoanalysis, dreams were a "safety valve" for the release of unconscious excitation that would otherwise awaken the individual. But his book made its mark less for how he thought dreams worked and more for what he thought they signified: disguised reflections of forbidden desires. His theory of dream interpretation captured the imagination of a sexually repressed Europe and became a metaphor for the dark side of human nature. Indeed, Freud's take permeated Western culture and wound up the dominant explanation for dreaming and a favorite trope in the arts.
But Freudian dream theory never gained any respectable scientific evidence and prevailed for a relatively brief 75 years. In 1977, Harvard psychiatrists Allan Hobson and Robert McCarley unveiled their alternative: the Activation-Synthesis model. Dreams, they postulated, arise from a "largely random and reflex process" during rapid-eye-movement (REM) sleep--the time when dreaming is most frequent and intense. Activation begins in the pontine brain stem and spreads up to the visual cortex, leading to internally generated imagery. This activation, "which is partially random and partially specific, is then compared with stored sensorimotor data in the synthesis of dream content." Thus, the properties of dreams--their bizarreness, visual vividness, frequent depictions of movement--all follow unavoidably from the neurophysiology and neurochemistry of REM sleep.
Hobson and McCarley attacked three key aspects of Freudian dream theory. First, they insisted that dreaming is a normal function of the sleeping brain, occurring every 90 minutes as the brain cycles in and out of REM sleep, and not an offshoot of neurosis. Second, they argued that dreams are instigated by random activations of the brain stem during REM sleep, not by a need to suppress unacceptable thoughts and desires. Third, they contended that the bizarre features of dreams are not due to any intentional obfuscation of some harbored illicitness; instead, "the forebrain may be making the best of a bad job in producing even partially coherent dream imagery from the relatively noisy signals sent up to it from the brain stem."
Hobson and McCarley also stressed that their model "does not deny meaning to dreams ... nor does it imply that they are without psychological meaning or function." Rather, psychological purpose derives from the synthesis portion of Activation-Synthesis. Therefore, in a comment particularly relevant today, "dreaming sleep may ... provide a biological model for the study of memory, [with] a functional role for dreaming sleep in promoting some aspect of the learning process."
Surprisingly, however, their evidence was circumstantial at best. The trailblazers based it on what they called "brain-mind isomorphisms"--similarities between psychological and physiological aspects of dreaming--an approach they credited to Freud. Hobson and McCarley suggest, then, that dreams of flying, for instance, ensue from the internal activation of the vestibular system during REM sleep. Like Kipling's Just So Stories, the logic makes sense, but the science leaves many gaps.
Nonetheless, the Activation-Synthesis model took off. In fact, the prestigious American Journal of Psychiatry published Hobson and McCarley's explication as a lead article. And Hobson, with whom I studied, later wrote extensively about the model, extolling the neurophysiological basis of dreaming, but focusing almost exclusively on activation. …