Cortical slow negative waves following non-paired stimuli: effects of modality, intensity and rate of stimulation

Using a simple stimulus counting task, negative after-waves following single (unpaired) stimuli were investigated under a variety of stimulus conditions. Responses were obtained to tones at 3 intensities, to light flashes at 3 intensities, and to tones at 3 rates of presentation. The acoustic stimuli led to a negative after-wave that peaked at frontal sites around 500 or 600 msec, and then trailed off to a more central scalp representation. This negative after-wave was increased in amplitude by slowing the rate of stimulus presentation. In comparison, no appreciable or sustained after-wave was elicited by visual stimuli. No significant effects in the negative after-wave were associated with intensity, either for visual or for acoustic stimuli. When analyzed by Principal Components Analysis, the negative after-waves were shown to comprise in all cases two underlying factors, although the factors contributed less to the total wave form for visual stimuli than for acoustic stimuli. Two interpretations for the negative after-wave were contrasted, one considering it to be an integral feature of the auditory evoked potential. A second interpretation, more compatible with the data obtained here, links the negative after-wave with non-specific activation processes.