Dynamic range for bone conduction ultrasound

Since Gavreau [1] reported that ultrasound could be perceived by bone conduction, controversial discussions have surrounded the concept of the auditory perception of ultrasound. Particular attention has been paid to its perception mechanism and to whether or not the profoundly hearingimpaired can detect ultrasound and discriminate ultrasound modulated by different speech sounds. Consideration is being given to the possibility of using ultrasound hearing aids as an alternative to cochlear implant. Pertaining to the peripheral mechanism of ultrasound perception, the following hypotheses were suggested by Dieroff and Ertelsu [2]. 1) Ultrasonic perception as a result of inadequate stimulation of particular sections of the hearing system [3]. 2) Ultrasonic perception as a result of adequate stimulation of particular sections of the hearing system. The second hypothesis was classified into two categories: the existence of rudimentary ultrasonic receptors [4,5] and the generation of subharmonic vibrations in the range of audio frequency through non-linearities in the transmission path of ultrasound. However, despite later studies, the peripheral mechanism of ultrasonic perception has not been clarified. There has been no report on the central mechanism of ultrasound perception, i.e., which sensory area in the brain is activated by ultrasound stimulation; the auditory area receiving it as a sound sensation, the somatosensory area processing it as a vibro-tactile sensation, or areas specific to ultrasound processing, before Hosoi et al. [6] reported that bone-conducted ultrasound could be perceived as a sound sensation. They observed that boneconducted ultrasound activated the auditory cortex generating the N1m component similar to that of normal sound with magnetoencephalography (MEG). They also measured the mismatch magnetic fields which reflect neural activities detecting acoustic differences, with ultrasound modulated by different speech sounds. Regarding the application of ultrasound to the hearing-impaired, Lenhardt et al. [7] reported that bone-conducted ultrasonic hearing was capable of supporting frequency discrimination and speech detection in not only normal but also elderly hearing-impaired and profoundly deaf subjects. They suggested that ultrasonic bone conduction hearing may potentially be used as an alternate communication channel in the rehabilitation of hearing disorders. In addition to such interesting issues, boneconducted ultrasound hearing has some characteristics different from normal sound perception. In this experiment, we have studied the effect of the intensity of stimuli and inter-stimulus intervals (ISI) for ultrasonic sensation as compared with normal sound sensation. We measured the amplitudes of N1m elicited by bone-conducted ultrasound and air-conducted normal sound stimuli using MEG in order to estimate the loudness growth in ultrasonic hearing.