Reverse learning

Reverse learning is a neurobiological theory of dreams. Crick and Mitchinson's (1983) reverse learning model likened the process of dreaming to a computer in that it was "off-line" during dreaming. During this phase, the brain supposedly sifts through information gathered throughout the day and throws out all unwanted material. According to the model, we dream in order to forget and this involves a process of "reverse learning."

The cortex cannot cope with the vast amount of information received throughout the day without developing "parasitic" thoughts that would disrupt the efficient organisation of memory. During REM sleep, these unwanted connections in cortical networks are wiped out by impulses bombarding the cortex from sub-cortical areas.

Support for the theory of reverse learning
In the echidna, a primitive egg-laying mammal that has no REM sleep, there is a very enlarged frontal cortex (Allison et al, 1972). Crick and Mitchison argue that this excessive cortical development is necessary to store both adaptive memories and parasitic memories, which in more highly evolved animals are disposed of during REM sleep.

Objections to the theory of reverse learning
One problem for reverse-learning theory is that dreams are often organized into clear narratives (stories). It is unclear why dreams would be organized in a systematic way if they consisted only of disposable parasitic thoughts. It is also unclear why babies sleep so much, because it seems they would have less to forget. Additionally, the brain of the echidna has far less folding than the brains of other mammals, so has less surface area (the location of the neo-cortex). It may actually have less capacity for higher thought than that of other mammals, rather than more, as it's greater mass suggests.

In response to these objections, Crick and Mitchinson restricted their theory to apply only to dreams with bizarre imagery and no clear narrative