According to the reverse learning theory, rapid eye movement (REM) sleep, the sleep stage that is associated with dreaming, prevents this from happening. During REM sleep, the neocortex's major connections to the external world are shut off. The brain stem then sends random stimuli to the neocortex. Crick and Mitchison hypothesized that this would cause some of the connections in the cortex to weaken, which would eliminate some of the thoughts and random associations that aren't useful to keep. They commented on the fact that dreams are often bizarre and illogical, like the expected outputs of parasitic memories.