Although the mechanisms involved remain unclear, several studies have reported that bone-conducted ultrasounds (BCUs) can be perceived even by those with profound sensorineural deafness, who typically hardly sense sounds even with conventional hearing aids. Interestingly, these patients appear to perceive BCUs as well as subjects with normal hearing. The perception of BCUs by the profoundly deaf has been objectively proven using magnetoencephalography (MEG). Furthermore, the author has identified both the psychological characteristics and the neurophysiological mechanisms underlying the perception of BCUs using psychophysical, electrophysiological, and physical approaches. In addition, the author has developed a BCU hearing aid (BCUHA) for the profoundly deaf. Remarkable results have already been achieved with this device, which enabled 42% of the profoundly deaf subjects in the trial to perceive some sort of sound and 17% of them to recognize simple words. These results suggest the feasibility of this device, but additional development and improvements are needed.
TopObjective Observation Of Bone-Conducted Ultrasonic Perception
First, we performed neurophysiological experiments and provided objective support for Lenhardt’s argument. Magnetoencephalography (MEG) (Hosoi, Imaizumi, Sakaguchi, Tonoike, & Murata, 1998; Nakagawa, Sakaguchi, Yamaguchi, Tonoike, Hosoi, Imaizumi, & Watanabe, 1999; Nakagawa, Yamaguchi, Tonoike, Hosoi, Imaizumi, Watanabe, 2000) and positron emission tomography (PET) (Imaizumi et al., 2001) revealed activation of the auditory cortex in response to BCUs both in profoundly deaf subjects and in subjects with normal hearing (Figure 1). Further, we showed that BCU amplitudes modulated by speech sounds were discriminated even by profoundly deaf subjects (Hosoi et al., 1998; Nakagawa et al., 1999) by measuring mismatch fields (MMFs), which generally reflect the perceptual properties of sound discrimination. These results support the potential of developing a BCUHA.
Figure 1. Auditory cortical activities evoked by BCUs in a profoundly deaf subject. Left: Auditory-evoked magnetic fields in response to 22, 27, 32, and 37-kHz bone-conducted tone-bursts. BCUs elicit prominent peaks at a latency of approximately 100 ms (N1m response). Right: Sources of the N1m responses to BCUs estimated in the auditory cortices.