Measurement of brain magnetic fields evoked by bone-conducted ultrasounds: effect of frequencies

Several studies have reported that high-frequency sounds of more than 20,000 Hz can be perceived by the profoundly sensorineural deaf subjects, as well as normal-hearing, through bone conduction. Although the mechanisms of how bone-conducted ultrasound (BCU) is perceived remain unclear, several characteristics that differ from air-conducted audible sounds are reported; for example, the pitch of BCU is independent of the frequency. In this study, the effects of BCU frequency on the activities of auditory cortex were investigated by magnetoencephalography (MEG). Auditory-evoked magnetic fields evoked by four kinds of BCU stimuli, 22,000, 27,000, 32,000, and 37,000 Hz tone bursts, were measured, and equivalent current dipoles (ECDs) of N1m were estimated. No significant differences were observed among the four kinds of BCU stimuli, not only in pitch, but also in N1m latency, ECD moment, and ECD location; whereas obvious differences were observed between the air-conducted sounds and BCUs. These results suggest that the mechanisms for BCU perception are different from those for air-conducted audible sounds.