Re-examination of the effects of periodic and aperiodic stimulation on the auditory-evoked vertex response.

It has been hypothesized that aperiodic stimulation will produce larger auditory-evoked vertex responses than periodic stimulation. Attentiveness toward an acoustic stimulus by human subjects has been shown in the past to enhance the auditory-evoked response. The unpredictability of aperiodic intersignal intervals has been assumed by previous investigators to produce more attentive listening behaviour and has therefore been used clinically to obtain 'enhanced' evoked responses. In order to examine this question further, auditory-evoked vertex responses were obtained with variable-interval (aperiodic) and with fixed-interval (periodic) schedules of stimulus presentation that had geometric mean intersignal intervals of 5 or 10 s. The evoked responses were obtained from normally-hearing adults while they were engaged in a counting task and while they were reading. Tone bursts were presented at 30 and 60 dB sensation level. No significant differences in peak-to-peak amplitude measures of components with latencies shorter than 300 ms could be attributed to aperiodic stimulation. When the time-base on the signal averager was extended to include components with latencies longer than 300 ms, no significant changes in component amplitudes could be attibuted to aperiodic stimulation, even though peak-to peak amplitudes during an attentive state (counting) were nearly double those obtained during an inattentive state (reading). It was concluded that aperiodicity in intersignal intervals is not a useful technique to employ in order to produce larger and more identifiable evoked responses for clinical diagnosis.