Brain slow potential changes elicited by missing stimuli and by externally paced voluntary responses.

Brain evoked potential changes were recorded from the scalp of 22 normal subjects and from the cortex of two patients in conditions where expected auditory stimuli were missing. It was found that such stimulus omissions could be used to initiate slow potential shifts (CNVs) in a warned foreperiod response task. Further slow potential shifts with durations of over 3 sec were recorded preceding self-initiated, but externally paced, responses made to one of a train of clicks. The experiments confirmed the cortical origins of the missing stimulus potentials, and illustrated the dependence of the CNV on endogenous factors rather than the physical properties of external stimuli. Contrary to most previous reports the negative component of the missing stimulus potential was in this study virtually identical in latency and amplitude to the N1 component of the click evoked potential. The slow potential shifts observed in the self-initiated condition contained elements of both the CNV and Bereitschaftspotential and appeared to reflect the steadily increasing cortical involvement during preparatory foreperiods.