During rapid eye movement (REM) sleep, the stage of sleep most closely connected with dreaming, a portion of the brain called the pons (located in the primitive hindbrain) generates electrical signals that go to many different brain areas, including those associated with motor activities, sensory activities, and conscious thought. Hobson and McCarley hypothesized that one of the effects of this electrical activity is to send a series of essentially random images, feelings, and so forth to the higher mental centers of the forebrain. This is the “activation” stage of the theory.
In normal waking consciousness, the fore- brain sorts through various kinds of internal and external sensory input to create a meaningful experience of the world. Faced with a barrage of disconnected inputs during REM sleep, the higher mental centers attempt to impose order on the incoming messages, creating whatever narrative structure dreams have. This is the “synthesis” stage of the theory. Many dreams are just masses of incoherent images representing incoming groups of signals that the brain was simply not able to synthesize.
For anyone who has been exasperated by the convolutions of Freudian or other schools of dream interpretation, the activation-synthesis theory has a certain iconoclastic appeal because it dismisses dreams as just so much nonsense. How- ever, because almost everyone has had at least a few truly insightful dreams, the theory is ultimately unsatisfying. Also, on a purely physiological level, it is an incomplete theory because it does not offer an explanation for the dreams that occur during non-REM sleep. [1]