Auditory frequency discrimination and event-related potentials

Auditory stimulus blocks were presented to 6 subjects. 80% of the stimuli in each block were standards of 1000 Hz and 20% were deviants of either 1002 Hz, 1004 Hz, 1008 Hz, 1016 Hz or 1032 Hz, one deviant type in each block. The constant interstimulus interval was 1 sec and the order of the stimuli was randomized. The subject was insructed either to ignore the deviant stimuli (ignore condition) or to press a response key to them (discrimination condition). In the ignore condition, an ERP component called the mismatch negativity (MMN), with a peak latency of approximately 170 msec, was elicited by those deviants exceeding the discrimination threshold (1016 Hz and 1032 Hz) and also those at the threshold (1008 Hz) tended to elicit a small MMN. In the discrimination condition, in addition to MMN, another negative component, N2b, was elicited by the detected deviants. This component had a somewhat longer latency than, and its midline distribution was posterior to, the MMN. The present results are in line with the hypothesis according to which the MMN component reflects the activation of cerebral mechanisms of passive discrimination, those which cause us to become aware of occasional changes in unattended stimulus sequences. In the discrimination condition, N2b and the slow parietal positivity were dominant features of the ERPs elicited by the detected suprathreshold deviants. The data obtained at the discrimination threshold specifically associate the parietal positivity with becoming aware of stimulus change since those deviants which were detected elicited this positivity whereas there was none to those (physically identical) deviants which remained undetected.