Prestimulus EEG-activity strongly influences the auditory evoked vertex response: a new method for selective averaging.

A new algorithm for averaging of evoked potentials (EPs) is introduced and compared with the conventional method. The root mean square (rms) values of the alpha and theta band EEG activities 1-s preceding stimulus presentation were computed: stimuli were blocked during phases of high prestimulus activity, thus evoking auditory responses selectively during periods of low activity. Ten volunteers were stimulated with tone bursts of 2000 Hz, 80 dB and 1-s duration. The main results were: (1) The averaged EPs showed an increase in amplitude of nearly 40% in comparison to experiments with conventional stimulation. This increase was in the range of 30% when an ISI correction was taken into account. (2) Single trials tended to form patterns of damped oscillations of e.g., 7-8 Hz, thus revealing a high correlation to the averaged EP. In alpha or theta band contingent experiments, correlation coefficients between single trials and averaged EPs were significantly higher (p < .01; p < .05).