Differential changes in frontal and sub-temporal components of mismatch negativity.

Novel evidence is reported showing changes in the mismatch negativity (MMN) event-related potential (ERP) that followed a different time course in frontal and sub-temporal (mastoid) electrodes. MMN recorded in frontal electrode sites, assumed to originate predominantly in the superior temporal gyrus, showed amplitude enhancement from the first to the second recording block. In contrast, the amplitude of the sub-temporal, mastoid component (termed mismatch positivity, MMP) diminished. In addition to these changes in the MMN elicited by tone duration deviants, there were also significant changes in the ERP to the standard stimulus. The positivity in the standard ERP between 50 and 150 ms increased in bilateral mastoid electrodes but showed no reliable change in frontal electrodes. This suggests that more than one generator underpins the two mismatch components, and that the sub-temporal mismatch component does not only represent a polarity reversal of the main MMN component in the superior temporal gyrus.