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 left by frequent standard stimuli. Therefore, the MMF amplitude has been thought to depend on the strength of such a trace. We examined this hypothesis in records with a 24-channel planar SQUID magnetometer by varying the number of stimuli preceding each deviant, the interdeviant interval (IDI) and the interstimulus interval (ISI) just preceding the deviant (pISI). When a constant IDI was employed and the number of standards between two deviants varied in different sessions, MMF amplitude increased as the number of standards increased. However, MMF did not depend on the number of standards between two deviants when the number 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 can 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 the stimulus sequence.