Sounds at frequencies of >24-kHz are classified as ultrasound which cannot be heard by humans if presented by air conduction, but can be perceived if presented by bone conduction. Some research studies involving ultrasonic hearing have reported that tinnitus is masked by bone-conducted ultrasound (BCU). However, little is known about residual inhibition (RI), which is a continuous reduction or disappearance of tinnitus after presentation of BCU. This study investigated whether RI could be induced by BCU. Five types of the masker sounds were used to measure RI in 21 subjects with tinnitus. A bone-conducted 30-kHz pure tone was used as a BCU, and an air-conducted 4-kHz pure tone, narrow-band noise, white noise, and a bone-conducted 4-kHz pure tone were used as controls of audible sounds. The masker intensities of the 30-kHz BCU and audible sounds were set at the minimum masking levels of tinnitus plus 3 and 10 dB, respectively, considering the narrow dynamic range of BCU. The duration of RI induced by the 30-kHz BCU was significantly longer than those induced by the 4-kHz sounds, but was not significantly different from that induced by the white noise. BCU activates the cochlear basal turn in response to the high frequency, which may broadly overlap with the frequency range that included the dominant tinnitus pitch in most of our subjects. The longer RI duration for the 30-kHz BCU was probably derived from this characteristic. These results suggested that the peripheral stimulation characteristic of BCU probably contributed to inducing long RI durations.
Keywords: Bandwidth; Central frequency; Dynamic range; Tinnitus pitch; Ultrasonic hearing.
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