Auditory evoked responses to unpredictable stimuli.

Abstract

ABSTRACTThe auditory evoked response (AER) to unpredictable stimuli was studied in 18 Ss. 100 msec sound bursts consisting of either a pure tone or white noise were presented every sec. One type of stimulus constituted the frequent expected stimulus and the other the infrequent stimulus that occurred as a random substitution. For the low probability (LP) stimulus condition, the mean ratio of infrequent to frequent was 1:30; for the intermediate probability (IP), 1:15; and for the high probability (HP), 1:7.5. Ss were instructed to ignore the sounds.The amplitude of a late positive wave (P3) of the AER was largest in the LP and smallest in the HP condition. There was a general decrease of all AER components over the course of a session. No evidence of dishabituation in the AER to the stimuli following the infrequent stimuli was obtained. The results of a detailed analysis of two orbital leads make it unlikely that eye movement or eye blink could account for the results.