After this period of Stage 2 sleep, rather than entering Stage 1 sleep, the subject progresses into a period of sleep characterized by rapid eye movements, known as REM (Myers 250). .
Brain wave activity during REM resembles that of normal wakefulness, however, the person is unmistakably asleep. During REM, the arousal threshold increases greatly and the brain stem paralyzes the body by blocking all signals from the motor cortex, leaving only the eyes free to move (Myers 251). If the subject is woken during this stage of sleep, they will more than 80% of the time report having been dreaming (Myers 251). Dreams do occasionally occur during other stages of sleep, however, these tend to consist of a single vague image whereas dreams occurring during REM tend to be more story-like (Myers 251). Using REM as an observable indicator of dreaming scientists have been able to conduct experiments that would have otherwise been impossible. .
Each cycle through the stages of sleep takes between 90 and 100 minutes to complete (Myers 251; "Dreaming"). The subject continues to cycle through these stages of sleep throughout the night, with Stages 4 and then 3 gradually disappearing. Periods of REM sleep however get progressively longer throughout the night (Myers 251).
Crick and Mitchison's proposal for the function of dreams has a basis not only in sleep research but also in neural network memory theory, therefore, it is necessary to also understand the underlying processes and functioning of neural networks in order to evaluate their theory. A neural network is an interconnected series of nodes, or neurons, which interact with each other through simple excitory signals. Each neuron has the possible states "+" or "-" analogous to a neuron in the brain firing or not firing. This state is determined by the total amount of excitory signals it is receiving from the neurons at any given moment (Hopfield, Feinstein and Palmer 158).
