Duration-dependent growth of N1m for speech-modulated bone-conducted ultrasound

Bone-conducted ultrasound (BCU) modulated by speech sound is recognized as speech sound and activates the auditory cortex similar to audible sound. To investigate the mechanisms of perception, the effects of stimulus duration on N1m were compared among air-conducted audible speech sound (AC speech), AC speech with carrier BCU and speech-modulated BCU in eight native Japanese with normal hearing. The Japanese vowel sound /a/ was used as a stimulus with durations of 10, 15, 20, 30, 40 and 60 ms. Comparison between AC speech with and without carrier showed that the presentation of carrier had no effect on N1m evoked by AC speech. Comparison among the three conditions showed that N1m amplitude for speech-modulated BCU differed from that for the two AC speeches. Moreover, N1m amplitude growth saturated at 40 ms for speech-modulated BCU, and at 20 ms for two AC speeches. These results suggest a difference in temporal integration of N1m between speech-modulated BCU and AC speech. Considering these results, it is reasonable to conclude that N1m evoked by speech-modulated BCU is influenced mainly by the ultrasonic component rather than demodulated audible sound. Given this finding, the notion needs to be considered that the mechanisms underlying perception and recognition of speech-modulated BCU depend on the ultrasonic component to some extent.