Activation Synthesis Theory is a neurobiological theory of dreams, put forward by Allan Hobson and Robert McCarley in 1977, which 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.

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.

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 synthesising 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 activation synthesis hypothesis would explain why we do not experience taste or smell in dreams as these neurons are not triggered. It is also supported by the fact we are paralysed in REM sleep.

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

Neuroscience