Introduction to the special section: Information processing during sleep onset and sleep

This Special Section examines the extent of information processing during s leep onset and sleep itself. It is generally agreed that, stimulus input is markedly inhibited during sleep, thus preventing conscious awareness of th e external environment. Overt behavioural responses are rarely made within sleep. Two neurophysiological measures are therefore often used. The electr ical activity of the brain (the EEG) can be employed to distinguish waking (conscious) from sleeping (unconscious) states. It is also possible to quan tify the EEG prior to and following a detection (or a failure of a detectio n) of a stimulus. Such measures can thus be used to predict conscious aware ness, A second measure that frequently has been employed is the brain's res ponse to an external stimulus (the evoked potential). Different components of the evoked potential can be used to trace the extent of information proc essing during the different states of consciousness. Some are associated wi th a preconscious detection while others are associated with conscious awar eness. Other evoked potentials may be unique to sleep.