Effects of selective attention on the late components of evoked potentials in man

Vertex and specific evoked potentials to visual and cutaneous stimuli were recorded in 6 subjects. The delivery of roughly equal numbers of flashes and shocks was paced by the subjects themselves. They also had to choose which stimulus they would be given next; 9 times out of 10 the stimulus matched the subject's expectation and stimuli from the unexpected modality were given randomly 1 time out of 10 on the average. No task was required after the stimulus occurrence. Cortical evoked potentials (CEPs) to unexpected stimuli were averaged and compared with CEPs the major difference consisted in large N2 and P3 waves in response to unexpected stimuli; no difference was observed in the amplitude of the N1 wave. In specific CEPs unexpected stimuli also elicited N2 and P3 waves; in addition N1 showed a slight tendency to be higher in CEPs to unexpected stimuli,