Perception Mechanism of Bone-Conducted Ultrasound and Its Clinical Use

Abstract

It is generally believed that ultrasound cannot be heard. However, ultrasound is audible when it is presented through bone conduction. Bone-conducted ultrasound (BCU) has unique characteristics; the most interesting is its perception in patients with profound deafness. Some patients can perceive it and discriminate speech-modulated BCU. Previous reports have suggested that BCU can be used for a hearing aid or tinnitus sound therapy. In this review, the perception of BCU at both the peripheral and central levels was investigated based on previous studies, although some of them remain controversial. We also investigated the clinical use of BCU. To develop hearing aids utilizing BCU, the encoding of speech signals into BCU has to be established. The outcomes of the reported speech modulations were evaluated. Furthermore, the suppression of tinnitus by BCU was reviewed, and the feasibility of the application of BCU to tinnitus treatment was investigated.

Keywords: bone conduction; high frequency sound; profound deaf; tinnitus; ultrasonic perception; ultrasound.

Figure 1

Schema of the excitation pattern and growth of masking by bone-conducted ultrasound (BCU) and air-conducted audible sound (ACAS). Parts (A,B) indicate the excitation pattern depending on the intensity. For BCU, the excitation does not show a sharp peak and spreads fast to the apical ward (Part (A)). For ACAS, the excitation pattern shows a peak at the characteristic frequency and spreads slowly to the apical ward (Part (B)). Part (C) indicates masking of ACAS by BCU. ACAS cannot be masked with a low-level masker (dashed line). However, when the excitation by BCU covers that by ACAS (solid line), ACAS is masked. Parts (D,E) indicate the growth of masking depending on the intensity. The dashed and solid lines indicate the masking pattern observed at a low- or high-level masker, respectively. The masking grows fast (Part (D)) and also spreads fast to the apical ward (Part (E)). In contrast, Part (F) indicates masking of BCU by ACAS. At low ACAS masker intensity (dashed line), the excitation pattern shows a sharp peak. Because BCU excites a broad cochlear region, ACAS below 60 dB SPL cannot mask BCU. When the masker intensity is sufficiently large (solid line), the excitation by ACAS covers a broad cochlear region and masks BCU. Parts (G,H) indicate the growth of masking, depending on the intensity. The dashed and solid lines indicate the masking at low and high signal intensities, respectively. The excitation by BCU fast spreads to the apical ward. Accordingly, a large increase in masker intensity (Part (G)) or masker frequency shift to the lower frequency range (Part (H)) is required for masking. In each schema, vertical and horizontal axes indicate the excitation and cochlear region, respectively. (Figure 1 was originally presented in Nishimura et al. 2011, Figure 1 [17]).

Figure 2

The average scores for the correct answers for each masker condition. The intelligibility of speech-modulated 30 kHz BCU was measured under five masking conditions in eight normal hearing volunteers. A numeral word list was used as speech materials. The ultrasonic transducer was fixed on the forehead. A speech-weighted noise was employed as masking and binaurally presented using earphones. The scores decreased as the masker intensity increased. The reduction in the scores showed variations among the stimuli. The vertical bars indicate standard deviations. (Figure 2 was originally presented in Nishimura et al. 2014, Figure 6 [39]).

Figure 3

Confusion matrices based on the results of speech-modulated bone-conducted ultrasound (A–E) and original speech signals (F–H) under different masking conditions. The results were obtained in the above-mentioned eight normal hearing volunteers. Presentation signals are represented on the left axis and responses across the top axis. “A” and “B” indicate other responses besides the six numeral words and no response, respectively. Blocks with larger grey values (i.e., darker shading) indicate higher appearance frequencies for those pairs. When the appearance frequency was higher than 50%, the block is marked fully black. Numerical values in the cells indicate the percentage of the appearance frequency. (Figure 3 was originally presented in Nishimura et al. 2014, Figure 10 [39]).