DETERMINANTS OF THE AUDITORY MISMATCH RESPONSE

The auditory mismatch field (MMF) is supposed to reflect a comparison process between an infrequent deviant stimulus and the memory trace le ft by frequent standard stimuli. Therefore, the MMF amplitude has been thought to depend on the strength of such a trace. We examined this h ypothesis in records with a 24-channel planar SQUID magnetometer by va rying the number of stimuli preceding each deviant, the interdeviant i nterval (IDI) and the interstimulus interval (ISI) just preceding the deviant (pISI). When a constant IDI was employed and the number of sta ndards between two deviants varied in different sessions, MMF amplitud e increased as the number of standards increased. However, MMF did not depend on the number of standards between two deviants when the numbe r varied within a single session and ISI varied as well. MMF decreased slightly when pISI increased from 0.6 to 3.4 sec. When IDI increased and the ISI remained constant, MMF amplitude increased. Most results c an be explained within the framework of the memory-trace hypothesis of MMF generation. However, the strengthening of the trace seems to be a complex process which is also affected by the temporal features of th e stimulus sequence.