Activation-synthesis hypothesis is a neurobiological theory of dreams forwarded by Harvard University psychiatrists James Allan Hobson and Robert McCarley, first published on the American Journal of Psychiatry in December of 1977. It states that dreams are a random event caused by firing of neurons in the brain. This random firing sends signals to the body's motor systems, but because of a paralysis that occurs during REM sleep, the brain is faced with a paradox. It synthesizes a narrative by drawing on memory systems in an attempt to make sense of what it has experienced.
To put it simply, Hobson and McCarley suggest that during REM sleep the cortex is highly active and activity in the brain triggers certain neurons at random (activation). The brain then tries to make sense of this by synthesizing the random impulses into what we experience as dreams, for example a cell triggering the area of the brain that controls balance may lead to a dream of falling. When we sleep, all sensory and motor input is blocked and neurons in the cerebral cortex are activated by random impulses; the forebrain then interprets this activity and produces the dream.
The original 1977 theory denied that dreams have meaning or are related to our real world environments. But this point drew criticism from other dream experts. In response, in 1988, Hobson published a revised theory acknowledging that dreams do reflect past memories, fears, hopes, and desires.
However, Hobson and McCarley rule out the idea that dreams may be caused by real events and are sometimes repeated over the period of a few nights and therefore cannot be completely random. Additionally, it has been argued by some that Hobson and McCarley's account of dreaming is incompatible with the phenomenon of lucid dreaming, though neuroimaging studies on the relationship of lucid dreaming to normal dreaming are still ongoing.