The electroencephalogram, or EEG, varies with the state of the sleep-wake cycle, disease, and other factors. EEG rhythms include alpha, beta, theta, and delta waves. The EEG reflects synaptic activity of pyramidal cells. Cortical evoked potentials are stimulus-triggered changes in the EEG, and they are useful clinical tests of sensory transmission. Slow wave sleep progresses through stages I through 4, each with a characteristic EEG pattern.
Most dreams occur in REM sleep.
Sleep is produced actively by a brainstem mechanism, and its circadian rhythmicity is controlled by the suprachiasmatic nucleus. The EEG helps in the recognition of the various forms of epilepsy. Seizures are associated with depolarization shifts in pyramidal cells. The left cerebral hemisphere is dominant for language in most individuals. Wernicke's area is responsible for the understanding of language and Broca's area for its expression.