The intensity of masking noise affects the mismatch negativity to speech sounds in human subjects

The present study investigated the effects of decreased audibility of the s peech sounds 'ba' and 'da' on the mismatch negativity (MMN). Audibility was altered by varying the intensity of the masking noise to be either 0 (no n oise), 65, 70 or 75 dB speech perception level. Cortical event-related pote ntials were recorded while normal listeners (n = 7) were instructed to igno re the stimuli. In a separate condition, subjects were asked to signal dete ction of the deviant stimulus by button-press, As audibility of speech soun ds was decreased, MMN peak amplitude decreased and MMN peak latency increas ed. Behavioral responses were in accordance with the MMN data. Hit rates de creased as audibility decreased. These results suggest that both pre-attent ive and attentive speech discrimination deteriorate following a reduction i n audibility. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.