While a monkey waits: electrocortical events recorded during the foreperiod of a reaction time study.

In a typical reaction time study, a stimulus is presented to the subject, and he is instructed to respond to that stimulus "as fast as he can". It has been well established that if this "imperative" stimulus is preceded by another whose function is to alert the subject that the imperative stimulus is about to arrive, his reaction time is considerably reduced. This reduction of the reaction time in forewarned subjects is not surprising, and yet until recently there were few data on the events that occur during the "foreperiod", as the interval between the warning and the imperative stimulus is called. We report here a study of the electrocortical events recorded in rhesus monkeys during the foreperiod. Our interest in the electrophysiology of the foreperiod has been triggered by the voluminous literature generated by Walter's report (Walter et al. 1964) that a slow negative wave can be recorded from certain areas of the scalp of human subjects during the foreperiod. This wave has been called by Walter the Contingent Negative Variation (CNV). Similar phenomena have been reported earlier by Kohler et al. (1952) and by