Cortical activities evoked by air- and bone-conducted sounds with frequency variations in an audible to ultrasonic range

Abstract

Abstract Few have reported the activities of auditory cortices to high-frequency sounds over 5000 Hz; though several studies have reported that bone-conducted ultrasounds (BCUs) are perceived even by those who are profoundly sensorineural deaf. Brain magnetic fields evoked by air- and bone-conducted sounds with frequency variations in an audible to ultrasonic range were recorded to clarify differences of perception mechanisms between (i) low- and high-frequency sounds, (ii) air- and bone-conducted sounds, and (iii) BCU and 12-kHz air-conducted sound, which had almost same pitch and loudness as BCU. Auditory-evoked magnetic fields evoked by air-conducted audible sounds (ACs; 500–16,000 Hz), bone-conducted audible sounds (BCs; 500–16,000 Hz), and a BCU (30,000 Hz) were measured and following results were obtained; (1) for all stimuli, substantial N1m responses were observed and equivalent current dipoles (ECDs) were localized in the Heschl's gyrus or the planum temporale. (2) Generally, the latencies increased and the ECD moments decreased as the frequency increased. (3) ACs and BCs produced by the same frequency sounds showed no differences in N1m latency and ECD moment below 8000 Hz; on the other hand, BCs showed larger latencies and smaller ECD moments than ACs over 12,000 Hz. (4) BCUs showed a larger latency and smaller ECD moments, and more posterior and lateral location than all ACs and BCs. Especially, BCU was different from 12-kHz AC in ECD moment, latency, and source location. (5) With all kinds of stimuli, the contralateral stimuli showed faster and larger N1m than the ipsilateral stimuli. These results suggest that air- and bone-conducted sounds are processed similarly below 8000 Hz, but high-frequency bone-conducted sounds over 12,000 Hz are processed differently. Considering the previous reports, the differences in the perception mechanisms possibly exists in the inner ear.