Neuroscience Letters 438 (2008) 260–262 Contents lists available at ScienceDirect Neuroscience Letters journal homepage: www.elsevier.com/locate/neulet Assessment of ability to discriminate frequency of bone-conducted ultrasound by mismatch fields Akinori Yamashita a,∗ , Tadashi Nishimura a , Seiji Nakagawa b , Takefumi Sakaguchi a , Hiroshi Hosoi a a Department of Otolaryngology, Nara Medical University, 840 Shijo-cho, Kashihara, Nara 634-8522, Japan b Institute for Human Science and Biomedical Engineering, National Institute of Advance Industrial Science and Technology (AIST), 1-8-31 Midorigaoka, Ikeda, Osaka 563-8577, Japan article info Article history: Received 25 June 2007 Received in revised form 21 December 2007 Accepted 30 March 2008 Keywords: Ultrasound Magnetoencephalography Bone conduction Mismatch fields Frequency modulation abstract According to previous studies, ultrasound can be perceived through bone conduction and ultrasound amplitude modulated by different speech sounds can be discriminated by some profoundly deaf subjects as well as the normal-hearing. These findings suggest the usefulness of development of a bone-conducted ultrasonic hearing aid (BCUHA) for profoundly deaf subjects. In this study, with a view to developing a frequency modulation system in a BCUHA, the capability to discriminate the frequency of sinusoidal bone-conducted ultrasound (BCU) was evaluated by measuring mismatch fields (MMF). We compared MMFs between BCU (standard stimuli were 30kHz, and deviant stimuli were 27 and 33kHz) and air- conducted audible sound (ACAS; standard stimuli were 1 kHz, and deviant stimuli were 900 and 1100 Hz). MMFs were observed in all subjects for ACAS, however, not observed in a few subjects for BCU. Further, the mean peak amplitudes of MMF for BCU were significantly less than those for ACAS. These findings indicate that the discrimination capability of frequency of sinusoidal BCU is inferior to that of ACAS. It was also demonstrated that normal hearing could to some extent discriminate differences in frequency in sinusoidal BCU. The results indicate a possibility of transmission system for language information making use of frequency discrimination. © 2008 Elsevier Ireland Ltd. All rights reserved. Generally speaking, human listeners perceive sound signals through air conduction from 20 to 20,000 Hz. Sound with fre- quency over 20 kHz is termed “ultrasound”. Until half a century ago, it was thought that ultrasound could not be perceived. How- ever, Gavreau reported in 1948 that ultrasound was audible when delivered by bone conduction [5]. Several researchers subsequently reported interesting perceptual characteristics of bone-conducted ultrasound (BCU), which differ markedly from those of audible sound. For instance, the subjective pitch of BCU is independent of its frequency [2,3] and it is perceived as if it were from air- conducted stimuli of 8–16 kHz [2,3,10]. In addition, BCU can mask perception of air-conducted audible sound (ACAS) of 10–14 kHz and this masking of ACAS is independent of ultrasonic frequency. The dynamic range of BCU is narrower than that of ACAS [9]. Interest- ingly, some profoundly deaf patients can hear BCU. Lenhardt et al. reported that BCU hearing supported frequency discrimination and speech detection in some deaf patients as well as normal-hearing ∗ Corresponding author. Tel.: +81 744 29 8887; fax: +81 744 24 6844. E-mail address: akinori@naramed-u.ac.jp (A. Yamashita). subjects by [7]. Furthermore, Hosoi et al. found using magnetoen- cephalography that BCU stimuli activate the auditory cortex, and that ultrasound amplitude modulated by different speech sounds can be discriminated in the auditory cortex in some profoundly deaf subjects [6]. These findings suggest the possibility of development of a bone-conducted ultrasonic hearing aid (BCUHA), which could be used by elderly hearing-impaired and profoundly deaf subjects [8]. For development of such a hearing aid, it is necessary to determine the most effective method of transmission of language information. Corso founded that BCU perception is characterized by poor frequency discrimination, although auditory perception in ACAS is characterized by excellent frequency discrimination [1]. This has been concluded from the finding that subjective pitch elicited by BCU stimulation is independent of its frequency and similar to that for the highest ACAS. Lenhardt, using a psychoa- coustical method, reported that just noticeable pitch differences in the ultrasonic range were on the order of 10% of the stimulus fre- quency, although that in the auditory range were between 0.4 and 1.0% [7]. These studies have found that ability to discriminate the frequency of BCU is inferior to that of ACAS. However, no study has 0304-3940/$ – see front matter © 2008 Elsevier Ireland Ltd. All rights reserved. doi:10.1016/j.neulet.2008.03.086