Assessments of basic properties of distal-presented bone-conducted ultrasonic hearing

Bone-conducted ultrasound (BCU) is perceived even by the profoundly sensorineural deaf and a novel hearing aid using the perception of amplitude-modulated BCU (BCU hearing aid) has been developed. In the BCU hearing aid, the vibrator is pressed onto a part of the cranial bone behind the ear (mastoid process). However, BCU can be heard on distal parts of the body; i. e., the muscle of the neck, the clavicle, and the upper limb. In this study, to assess basic properties of such distal-presented BCU hearing, hearing thresholds were measured when 30-kHz tone bursts were presented to the neck and the upper and lower arms in normal hearing participants. Further, to assess the basic capability of transferring information by the distal-presented BCU hearing, temporal modulation transfer functions (TMTFs), that reflect the temporal resolution of the hearing, were estimated for 30-kHz carrier. The results showed that BCUs presented to the distal parts, including the lower arm, can be perceived at least in the normal hearing, whereas threshold increased depending on the distance from the head. Also, the temporal resolutions of the distal-presented BCU hearing at the neck, and the upper and lower arms were comparable to that of the mastoid process of the temporal bone. These results provide useful information not only for the improvement of the existing BCU hearing aid, but also for the development of novel distal-presented BCU devices that can provide sound information selectively to the specific person who touches the device by the arms or so.