Temporal window of integration estimated by omission in bone-conducted ultrasound.

Bone-conducted ultrasound (BCU) can be heard for both normal-hearing and some profoundly deaf individuals. Moreover, amplitude-modulated BCU can transmit the speech signal. These characteristics of BCU provide the possibility of the developing a bone-conducted ultrasonic hearing aid. Previous studies on the perception mechanism of speech-modulated BCU have pointed to the importance of temporal rather than frequency information. In order to elucidate the perception of speech-modulated BCU, further investigation is need concerning the processing of temporal information. The temporal processing of air-conducted audible sounds (ACASs) involves the integration of closely presented sounds into a single information unit. The long-temporal window of integration was estimated approximately 150-200 ms, which contribute to the discrimination of speech sound. The present study investigated the long-temporal integration system for BCU evaluated by stimulus omission using magnetoencephalography. Eight participants with normal hearing took part in this study. Ultrasonic tone burst with the duration of 50 ms and frequency of 30 kHz was used as the standard stimulus and presented with steady onset-to-onset times or stimulus-onset asynchronies (SOAs). In each sequence, the duration of the SOAs were set to 100, 125, 150, 175, 200, or 350 ms. For deviant, tones were randomly omitted from the stimulus train. Definite mismatch fields were elicited by sound omission in the stimulus train with an SOA of 100-150 ms, but weren't with an SOA of 200 and 350 ms for all participants. We found that stimulus train for BCUs can be integrated within a temporal window of integration with an SOA of 100-150 ms, but are regarded as a separate event when the SOA is 200 or 350 ms in duration. Therefore, we demonstrated that the long-temporal window of integration for BCUs estimated by omission was 150-200 ms, which was similar to that for ACAS (Yabe et al. NeuroReport 8 (1997) 1971-1974 and Psychophysiology. 35 (1998) 615-619). These findings contribute to the elucidation and improvement of the perception of speech-modulated BCU.

Perception of speech in cartilage conduction.

OBJECTIVE: By attaching a transducer to the aural cartilage a relatively loud sound is audible even with a negligibly small fixation pressure applied to the transducer. This form of conduction is referred to as cartilage conduction (CC). Utilizing CC, novel audio devices can be developed, and one possible application is a CC hearing aid. However, there are no studies on speech perception in CC. In this study, CC speech recognition performance was measured and compared with that for air and bone conduction (AC and BC, respectively). METHODS: Nine volunteers with normal hearing participated in the study. The performance-intensity functions were measured for AC, BC and CC. These measurements were performed in the conditions with and without an earplug. RESULTS: Without the earplug, no differences in speech recognition scores were observed among AC, BC, and CC. With the earplug, the level at which the maximum speech recognition score was obtained did not increase in CC, which agreed with the result of BC but not AC. The maximum speech recognition CC score decreased with the earplug. The performance-intensity functions for AC and BC shifted in parallel with the earplug. These shifts approximated the average threshold shifts. In contrast, for CC, the performance-intensity function did not shift in parallel with the earplug. As for the CC threshold shifts with the earplug, although the threshold at 500Hz decreased by 15.4dB, those at 2000 and 4000Hz increased by 13.8 and 31.1dB, respectively. Compared with AC and BC, CC excessively emphasized low over high frequency sounds when the earplug was inserted. Confusion matrices analysis demonstrated that 4%, 22%, and 74% of the errors occurred at low, intermediate, and high frequency speech sounds, respectively. Thus, this excessive low frequency sound emphasis probably prevented the recognition of high frequency speech sounds. CONCLUSION: The decrease in the maximum speech recognition score for CC with the earplug was derived from the biased frequency composition. It can be improved by frequency composition adjustment.

Long-term effects of electrotactile sensory substitution therapy on balance disorders.

This clinical research investigated whether a new type of rehabilitation therapy involving the use of a vestibular substitution tongue device (VSTD) is effective for severe balance disorders caused by unilateral vestibular loss. Sixteen patients with postural imbalances because of unilateral vestibular loss underwent training with VSTD. The VSTD transmits information on the head position to the brain through the tongue as substitutes for the lost vestibular information. The device's electrode array was placed on the tongue and participants were trained to maintain a centered body position by ensuring the electrical signals in the center of their tongue. All participants completed 10 min training sessions 2-3 times per day for 8 weeks. Functional gait assessments and the dizziness handicap inventory were, respectively, used to the evaluate participants' dynamic gait function and their severity of balance problems before and after the training period. All examined parameters improved after the 8-week training period. These changes were maintained for up to 2 years after the termination of the training program. Short-term training with VSTD had beneficial carry-over effects. VSTD training might represent a useful rehabilitation therapy in individuals with persistent balance disorders and might lead to long-term improvements in their balance performance and ability to perform daily and social activities.

An examination of the effects of broadband air-conduction masker on the speech intelligibility of speech-modulated bone-conduction ultrasound.

Ultrasound can be heard by bone-conduction, and speech-modulated bone-conducted ultrasound (BCU) delivers the speech information to the human ear. One of the recognition mechanisms is the demodulation of the signals. Because some of the profoundly deaf can also hear speech-modulated BCU, another mechanism may also contribution to the recognition of speech-modulated BCU. In this study, eight volunteers with normal hearing participated. The intelligibilities of speech-modulated BCU were measured using a numeral word list under masking conditions. Because the masker can mask the demodulated sounds, the evaluation of the masking reveals the contribution of the demodulation to the recognition of speech-modulated BCU. In the current results, the masking of speech-modulated BCU differed from that of original non-modulated speech. Although the masking shifted the recognition curve for the original speech upward, the same results were not observed for the speech-modulated BCU. The masking generated the difference in the correct answers among the words for the speech-modulated BCU. The current results suggested the importance of the envelope of the modulated ultrasonic signal to the recognition under masking condition. Both demodulation and direct ultrasonic stimulation contribute to the recognition of speech-modulated BCU for the normal hearing individuals, and the direct ultrasonic stimulation plays an important role in the recognition for the profoundly deaf.

Cartilage conduction hearing.

Sound information is known to travel to the cochlea via either air or bone conduction. However, a vibration signal, delivered to the aural cartilage via a transducer, can also produce a clearly audible sound. This type of conduction has been termed "cartilage conduction." The aural cartilage forms the outer ear and is distributed around the exterior half of the external auditory canal. In cartilage conduction, the cartilage and transducer play the roles of a diaphragm and voice coil of a loudspeaker, respectively. There is a large gap between the impedances of cartilage and skull bone, such that cartilage vibrations are not easily transmitted through bone. Thus, these methods of conduction are distinct. In this study, force was used to apply a transducer to aural cartilage, and it was found that the sound in the auditory canal was amplified, especially for frequencies below 2 kHz. This effect was most pronounced at an application force of 1 N, which is low enough to ensure comfort in the design of hearing aids. The possibility of using force adjustments to vary amplification may also have applications for cell phone design.

Residual inhibition of tinnitus induced by 30-kHz bone-conducted ultrasound.

Sounds at frequencies of >24-kHz are classified as ultrasound which cannot be heard by humans if presented by air conduction, but can be perceived if presented by bone conduction. Some research studies involving ultrasonic hearing have reported that tinnitus is masked by bone-conducted ultrasound (BCU). However, little is known about residual inhibition (RI), which is a continuous reduction or disappearance of tinnitus after presentation of BCU. This study investigated whether RI could be induced by BCU. Five types of the masker sounds were used to measure RI in 21 subjects with tinnitus. A bone-conducted 30-kHz pure tone was used as a BCU, and an air-conducted 4-kHz pure tone, narrow-band noise, white noise, and a bone-conducted 4-kHz pure tone were used as controls of audible sounds. The masker intensities of the 30-kHz BCU and audible sounds were set at the minimum masking levels of tinnitus plus 3 and 10 dB, respectively, considering the narrow dynamic range of BCU. The duration of RI induced by the 30-kHz BCU was significantly longer than those induced by the 4-kHz sounds, but was not significantly different from that induced by the white noise. BCU activates the cochlear basal turn in response to the high frequency, which may broadly overlap with the frequency range that included the dominant tinnitus pitch in most of our subjects. The longer RI duration for the 30-kHz BCU was probably derived from this characteristic. These results suggested that the peripheral stimulation characteristic of BCU probably contributed to inducing long RI durations.

Evaluation of prosodic and segmental change in speech-modulated bone-conducted ultrasound by mismatch fields.

Speech-modulated bone-conducted ultrasound (BCU) can transmit speech sounds for some profoundly deaf individuals. Hearing aids using BCU are considered to be a novel hearing system for such individuals. In our previous study, the speech discrimination for speech-modulated BCU was objectively confirmed using a magnetoencephalography. Moreover, in our previous behavioral study, prosodic information for speech-modulated BCU could also be discriminated in the normal hearing. However, the prosodic discrimination for speech-modulated BCU has not objectively been studied. In order to evaluate the prosodic discrimination for speech-modulated BCU, mismatch fields (MMFs) elicited by prosodic and segmental change were measured for speech-modulated BCU and air-conducted speech. Ten Japanese participants with normal hearing took part in this study. Stimuli re-synthesized from the speech of a native Japanese female adult were used. Standard stimulus was /itta/ with a flat pitch pattern, and two deviant stimuli were /itta?/ with a rising pitch pattern and /itte/ with a flat pitch pattern. All and nine participants elicited the prominent MMF elicited by the prosodic and segmental change for the speech-modulated BCU, respectively. The moment of MMF components for speech-modulated BCU was significantly smaller than those for air-conducted speech, while no difference in the MMF latency elicited by the prosodic and segmental change were observed between both stimulus conditions. Comparing the MMFs elicited by prosodic and segmental change, no significant differences were observed for both stimulus conditions. Thus, it is suggested that the prosodic change can be discriminate to the same degree as segmental change even for speech-modulated BCU. However, discrimination capability for speech-modulated BCU is slightly inferior to that for air-conducted speech.