Prestimulus spectral EEG patterns and the evoked auditory vertex response.

A vigilance-related index (slow wave index, SWI) representing the relative delta + theta amplitude of the 1 sec prestimulus EEG spectrum has been related to latencies and amplitudes of the auditory cortical evoked response (EP) in 12 volunteers. Stimuli were 70 dB SL tone bursts delivered at 10 sec ISI. Four selective averages were obtained for each subject according to the subdivision of the SWI into quartiles. The subjects were divided into 2 groups having individual mean SWI values above or below the group's median SWI. Repeated measures MANOVA showed that N1 latency increased within subjects from the first to the fourth SWI quartile and that it was longer in subjects with high mean SWIs. N1-P2 amplitude decreased within subjects from the first to the fourth SWI quartile, but only in the group with high mean SWI.