Seiji Nakagawa

Perception Mechanisms of Bone-Conducted Ultrasound Assessed by Acoustic Characteristics in the External Auditory Meatus

Acoustic fields/vibrations in the external auditory meatus (ear canal) and tympanic membrane (TM) under bone-conducted ultrasonic stimulation were measured to elucidate the contributions of the osseotympanic and inertial routes to bone-conducted ultrasonic (BCU) perception. Evidence showing nonlinear distortions, especially the generation of audible subharmonics in the outer and middle ear, was examined. In the results, we did not find any audible signals corresponding to the subjective pitch of a BCU tone in the acoustic fields for the auditory meatus and TM vibrations. This suggests that nonlinear distortions in the osseotympanic and inertial routes do not contribute to bone-conducted ultrasonic perception. Specific properties of perception may be related to mechanisms in the cochlea or afferent neural pathway.

Modulation detection for amplitude-modulated bone-conducted sounds with sinusoidal carriers in the high- and ultrasonic-frequency range

Ultrasonic vibration generates a sensation of sound via bone-conduction. This phenomenon is called bone-conducted ultrasonic (BCU) hearing. Complex sounds can also be perceived by amplitude-modulating a BCU stimulus (AM-BCU). The influence of the modulation frequency on the sensitivity to detecting amplitude modulation of sinusoidal carriers of 10, 20, and 30 kHz was examined to clarify the characteristics of the perception of amplitude modulation over the sonic or audio-frequency range and the ultrasonic range. In addition, the detection sensitivity for single-sideband modulation for a 20 kHz carrier was measured. Temporal modulation transfer functions (TMTFs) obtained at each carrier frequency suggest that the auditory system has the ability to process timing information in the envelopes of AM-BCUs at lower modulation frequencies, as is the case with audio-frequency sounds. The possible influence of peripheral filtering on the shape of the TMTF at higher frequencies was examined.

Duration-dependent growth of N1m for speech-modulated bone-conducted ultrasound

Bone-conducted ultrasound (BCU) modulated by speech sound is recognized as speech sound and activates the auditory cortex similar to audible sound. To investigate the mechanisms of perception, the effects of stimulus duration on N1m were compared among air-conducted audible speech sound (AC speech), AC speech with carrier BCU and speech-modulated BCU in eight native Japanese with normal hearing. The Japanese vowel sound /a/ was used as a stimulus with durations of 10, 15, 20, 30, 40 and 60 ms. Comparison between AC speech with and without carrier showed that the presentation of carrier had no effect on N1m evoked by AC speech. Comparison among the three conditions showed that N1m amplitude for speech-modulated BCU differed from that for the two AC speeches. Moreover, N1m amplitude growth saturated at 40 ms for speech-modulated BCU, and at 20 ms for two AC speeches. These results suggest a difference in temporal integration of N1m between speech-modulated BCU and AC speech. Considering these results, it is reasonable to conclude that N1m evoked by speech-modulated BCU is influenced mainly by the ultrasonic component rather than demodulated audible sound. Given this finding, the notion needs to be considered that the mechanisms underlying perception and recognition of speech-modulated BCU depend on the ultrasonic component to some extent.

Temporal integration affects intensity change detection in human auditory cortex.

The aim of this paper was to clarify the effects of temporal integration on the auditory evoked response elicited by sounds of varying intensity. We measured auditory evoked fields in response to tones with different intervals of intensity change. The amplitude of the N1m′, occurring approximately 100 ms after the intensity change, remained constant when the interval was longer than 250 ms. The recovery function we observed suggests that the neural populations underlying the N1m′ are close to those underlying the late anterior N1m component. The N1m′ amplitude decreased with decreasing intervals of intensity change at intervals less than 250 ms. This finding supports the notion that a disinhibitory process is caused by the offset of the sound.

Body-conducted speech retrieval from a noisy environment using differential acceleration

Speech recognition tends to be influenced by noise in the air. Body-conducted speech offers a robust signal extraction method from noisy environments, particularly because body-conducted speech is a propagated sound and not easily influenced by noise. However, when body-conducted speech is extracted with an accelerator, the typical frequency component of 2 kHz or more decreases compared with normal speech. Thus, we investigated a technique combining differential acceleration and noise reduction to estimate a clear signal using only body-conducted speech from a noisy room. The recognition rate confirmed about 3-4% is improved with proposed method.

Objective Assessments of Bone-Conducted Ultrasonic (BCU) Hearing by Auditory Evoked Fields

Bone-conducted ultrasound (BCU) is perceived even by the profoundly sensorineural deaf. We have been developing a novel hearing-aid using BCU perception, which transmits a 30-kHz bone-conducted carrier that is amplitude- modulated by speech. The fundamental capabilities of the BCU hearing-aid (BCUHA) were objectively assessed by magneto- encephalography. Frequency- and temporal-resolution were assessed by measuring mismatch fields (MMFs). As well, the capability to discriminate two-channel BCUs presented to the left and right mastoids was verified by the laterality of auditory evoked magnetic field N1m. In the frequency- discrimination tests, amplitude-modulated BCU (AM-BCU) and air-conducted sound (AC) showed almost the same MMF- magnitudes when frequency/modulation-frequency of the standard-stimuli were 1.0 kHz, even though MMF-magnitudes for BCU were smaller than those of AC at 0.125 kHz. In the temporal-discrimination tests, MMFs for changes of stimulus- duration were measured. MMF-magnitudes for AM-BCUs reached more than 80% of those for air-conducted sounds (ACs). In addition, in the laterality-discrimination tests, N1m evoked by contralateral stimuli were larger in amplitude and shorter in latency than those evoked by ipsilateral stimuli for both AM-BCUs and ACs. This result agrees with earlier reports of AC auditions, and suggests that two-BCU channels presented to the left and right mastoid were separately localized. These findings objectively indicate that the BCUHA has practical frequency- and temporal resolution, and give a rationale for the development of a multi-channel (at least two) BCUHA.