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 speech 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 noise), 65, 70 or 75 dB speech perception level. Cortical event-related potentials were recorded while normal listeners (n = 7) were instructed to ignore the stimuli. In a separate condition, subjects were asked to signal detection of the deviant stimulus by button-press, As audibility of speech sounds was decreased, MMN peak amplitude decreased and MMN peak latency increased. Behavioral responses were in accordance with the MMN data. Hit rates decreased as audibility decreased. These results suggest that both pre-attentive and attentive speech discrimination deteriorate following a reduction in audibility. (C) 2001 Elsevier Science Ireland Ltd. All rights reserved.