Perception mechanisms of bone-conducted ultrasound assessed by electrophysiological measurements in humans

Several studies have reported that bone-conducted ultrasound (BCU) is even perceived by the profoundly sensorineural deaf; however, the mechanisms involved remain unclear. We previously reported some unique characteristics of BCU perception that suggest unique neurophisiological mechanisms. For example: (1) the pitch of BCU is 10-odd kHz and is independent of its frequency; (2) no beats occur between BCU and 10-odd-kHz air-conducted sinusoids; (3) BCU masks 10-14 kHz air-conducted sounds; and (4) the dynamic range of loudness for BCU is less than 20 dB. In the present study, several electrophysiological measurements were carried out to clarify the neural pathway involved in BCU perception. Specifically, auditory brainstem responses (ABRs), middle latency responses (MLRs), and cortical magnetic activities evoked by BCU were measured in humans. Substantial ABRs (wave I -V) and MLRs were evoked by BCU as well as air-conducted audible sound. In terms of cortical activities, clear N1m responses were elicited by BCU, with their sources being localized in the auditory cortices. However, BCU showed later, smaller, more posterior, and more lateral sources than audible-frequency sounds, and therefore did not appear to follow a tonotopic organization at the cortical level. These results indicate that: (1) the cochlea substantially contributes to BCU perception; (2) BCU goes through the normal auditory pathway and there is no special organ for BCU perception; and (3) some differences in the inner ear mechanisms may exist between audible sounds and BCU.