Lateralization of Amplitude-modulated Bone-conducted Ultrasounds: Discrimination of Interaural Time and Intensity Differences and Time-intensity Trading

Ultrasonic vibration generates the sensation of sound via bone-conduction; the phenomenon is called bone-conducted ultrasonic hearing. Complex sounds also can be perceived by amplitude-modulated bone-conducted ultrasound (AM-BCU). In this study, three experiments were conducted to examine whether interaural time differences in the amplitude envelopes (envelope-ITDs) and interaural intensity differences (IIDs) can be used as cues for lateralization of bilaterally applied AM-BCUs. These were discrimination experiments of envelope-ITDs, IIDs and their combination. AM-BCU stimuli were created by modulating a 30-kHz sinusoid with lowpass noises with various cut-off frequencies. The results showed that polarities of envelope-ITDs or IIDs on bilateral AM-BCUs were discriminated, i.e., envelope-proceeding sides or intensity-exceeding sides of bilateral AMBCUs were detected. Also, the discrimination thresholds of the polarities of IIDs were compensated by envelope-ITDs, i.e., the time-intensity trading was observed in the AM-BCU perception. These findings indicate that the auditory system has the ability for lateralization using envelope-ITDs and IIDs of bilateral AM-BCUs. Further, it is suggested that bilateral AMBCUs are processed in the auditory pathway associated with lateralization in a similar manner to high-frequency amplitude-modulated sounds.