The effects of decreased audibility produced by high-pass noise masking onN1 and the mismatch negativity to speech sounds /ba/ and /da/

This study investigated the effects of decreased audibility produced by hig h-pass noise masking on the cortical event-related potentials (ERPs) N1 and mismatch negativity (MMN) to the speech sounds /ba/ and /da/, presented at 65 dB SPL. ERPs were recorded while normal listeners (N = 11) ignored the stimuli and read a book. Broadband masking noise was simultaneously present ed at an intensity sufficient to mask the response to the speech sounds, an d subsequently high-pass filtered. The conditions were QUIET (no noise); hi gh-pass cutoff frequencies of 4000, 2000, 1000, 500, and 250 Hz; and broadb and noise. Behavioral measures of discrimination of the speech sounds (d' a nd reaction time) were obtained separately from the ERPs for each listener and condition. As the cutoff frequency of the high-pass masker was lowered, ERP latencies increased and amplitudes decreased. The cutoff frequency whe re changes first occurred differed for N1 and MMN. N1 showed small systemat ic changes across frequency beginning with the 4000-Hz high-pass noise. MMN and behavioral measures showed large changes that occurred at approximatel y 1000 Hz. These results indicate that decreased audibility resulting from the masking, affects N1 and the MMN in a differential manner. N1 reflects t he presence of audible stimulus energy, being present in all conditions whe re stimuli were audible, whether or not they were discriminable. The MMN is present only For those conditions where stimuli were behaviorally discrimi nable. These studies of cortical ERPs in high-pass noise studies provide in sight into the changes in brain processes and behavioral performance that o ccur when audibility is reduced, as in hearing loss.