Objective Assessments of Bone-Conducted Ultrasonic (BCU) Hearing by Auditory Evoked Fields

Bone-conducted ultrasound (BCU) is perceived even by the profoundly sensorineural deaf. We have been developing a novel hearing-aid using BCU perception, which transmits a 30-kHz bone-conducted carrier that is amplitude-modulated by speech. The fundamental capabilities of the BCU hearing-aid (BCUHA) were objectively assessed by magnetoencephalography. Frequency- and temporal-resolution were assessed by measuring mismatch fields (MMFs). As well, the capability to discriminate two-channel BCUs presented to the left and right mastoids was verified by the laterality of auditory evoked magnetic field N1m. In the frequency-discrimination tests, amplitude-modulated BCU (AM-BCU) and air-conducted sound (AC) showed almost the same MMF-magnitudes when frequency/modulation-frequency of the standard-stimuli were 1.0 kHz, even though MMF-magnitudes for BCU were smaller than those of AC at 0.125 kHz. In the temporal-discrimination tests, MMFs for changes of stimulus-duration were measured. MMF-magnitudes for AM-BCUs reached more than 80% of those for air-conducted sounds (ACs). In addition, in the laterality-discrimination tests, N1m evoked by contralateral stimuli were larger in amplitude and shorter in latency than those evoked by ipsilateral stimuli for both AM-BCUs and ACs. This result agrees with earlier reports of AC auditions, and suggests that two-BCU channels presented to the left and right mastoid were separately localized. These findings objectively indicate that the BCUHA has practical frequency- and temporal resolution, and give a rationale for the development of a multi-channel (at least two) BCUHA.