The ERP signature of sound rise time changes

Sounds, whether speech or non-speech, vary in how rapidly their peak amplitudes are reached. The time taken for sounds to reach their maximum amplitude is known as the rise time and this variable is an important perceptual cue for representation of the sound amplitude envelope. In the experiments described here healthy adult volunteers passively listened to tones varying in rise time. An oddball design was used and both N1 and MMN components were examined. Experiment 1 contrasted tone stimuli of 15 ms vs. 185 ms rise time. The shorter 15 ms rise time stimuli elicited an N1b over central frontal electrodes of significantly greater amplitude than the 185 ms rise time stimuli. MMN was also observable for both the 15 ms and 185 ms rise time tones when the same stimuli served as deviant vs. standard. Experiment 2 explored the possible confound of rise time and overall stimulus intensity change (tones with shorter rise times sound louder). New stimuli were created in which overall stimulus intensity between short and long rise times was perceptually matched. N1b amplitude differences to the contrastive rise times were still observed, suggesting that N1b may reflect an auditory cortex detector mechanism sensitive to changes in rise time, relatively independently of sound intensity changes. These findings are discussed with reference to their implications for speech perception processes.