中川 誠司

Elastic wave scattering off a layer containing a single set of vertical periodic fractures is examined using a numerical technique based on the work of Hennion et al. [J. Acoust. Soc. Am. 87, 1861-1870 (1990)]. This technique combines the finite element method and plane wave method to simulate three-dimensional scattering off a two-dimensional fractured layer structure. Each fracture is modeled explicitly, so that the model can simulate both discrete arrivals of scattered waves from individual fractures and multiply scattered waves between the fractures. Using this technique, we examine changes in scattering characteristics of plane elastic waves as a function of wave frequency, angle of incidence, and fracture properties such as fracture stiffness, height, and regular and irregular spacing.

Auditory sensation is affected by a forward masker, and this phenomenon has been demonstrated in a neural adaptation model and a temporal window (integration) model. To study forward masking in the central auditory system, the growth of the N1m amplitude was measured by varying the signal delay. In the adaptation model, the masking increases as the signal delay decreases. However, in our results, the miN1mum N1m amplitude was observed at a signal delay of 40 ms. As the signal delay decreased from 40 ms, the N1m amplitude increased although the masking increased. Our results suggest that the growth of the N1m amplitude largely depends on temporal integration at signal delays below 40 ms.

Electrogustometry is a convenient method to examine taste acuity in clinical situations. Some basic properties of neural activity in the cerebral cortex in response to electrogustatory stimulation were revealed by measuring magnetoencephalography (MEG) signals with a whole-cortex-type system in response to varying intensities of anodal DC currents focally applied to the tongue surface in human subjects. Independent component analysis was used to eliminate stimulus artifacts in MEG signals. Electrogustatory stimulation with intensities of induced electric taste evoked responses bilaterally, mainly in the opercular-insular cortex with a mean onset latency of approximately 350 ms, while subthreshold electrogustatory stimulation induced modest responses in the cortex. Stronger stimulation induced a tingling sensation and elicited large transient responses in both the opercular-insular and somatic sensory cortices. This is the first description of the basic properties of human MEG responses to electrogustatory stimulation.

Bone conduction enables ultrasound to be heard. Although several hypotheses about ultrasonic perception have been presented, the perception mechanism of bone-conducted ultrasound has not yet been established. In this study, to investigate ultrasonic perception, the amount of masking produced by 27-, 30- and 33-kHz bone-conducted ultrasonic maskers for air-conducted high-frequency sounds was measured in the frequency range of 8-18 kHz at 1-kHz intervals. The results showed that the air-conducted signals in the frequency range of 10-14 kHz were strongly masked by the ultrasonic maskers. When the masker intensity increased from 5 to 10 dB SL, the growth of masking was more than 10 dB in the frequency range of 9-15 kHz, and the masking spread strongly to lower frequencies. Furthermore, the dynamic range for bone-conducted ultrasound was clearly narrower than that for air-conducted high-frequency sounds. These results suggest that perception of bone-conducted ultrasound depends on inner hair cell activity induced by ultrasound, even without modulation being present, and does not depend on enhancement by the outer hair cells in the basal turn of the cochlea.

Bone conduction enables ultrasound to be heard. Although several hypotheses about ultrasonic perception have been presented, the perception mechanism of bone-conducted ultrasound has not yet been established. In this study, to investigate ultrasonic perception, the amount of masking produced by 27-, 30- and 33-kHz bone-conducted ultrasonic maskers for air-conducted high-frequency sounds was measured in the frequency range of 8-18 kHz at 1-kHz intervals. The results showed that the air-conducted signals in the frequency range of 10-14 kHz were strongly masked by the ultrasonic maskers. When the masker intensity increased from 5 to 10 dB SL, the growth of masking was more than 10 dB in the frequency range of 9-15 kHz, and the masking spread strongly to lower frequencies. Furthermore, the dynamic range for bone-conducted ultrasound was clearly narrower than that for air-conducted high-frequency sounds. These results suggest that perception of bone-conducted ultrasound depends on inner hair cell activity induced by ultrasound, even without modulation being present, and does not depend on enhancement by the outer hair cells in the basal turn of the cochlea. (C) 2002 Elsevier Science B.V. All rights reserved.

Cortical site processing the information of whole body linear acceleration has not yet been identified. In this study, neuromagnetic responses to visually induced linear forward acceleration were recorded in six healthy-right-handed adult subjects using a 122-channel whole cortex neuromagnetometer. Significant activation was estimated in the cortex around the posterior insula, which belongs to the vestibular cortex. Hence, it is suggested that the vestibular cortex not only receives vestibular input from the peripheral vestibular apparatus, but also processes the vestibular sensation from multi-modal information.

Ultrasound can be heard by bone conduction in man. However, there has been no consensus about the perception mechanism of bone-conducted ultrasound (BCU). In the current study, to clarify the central auditory system of BCU, the effects of stimulus duration for 30 kHz BCU on N1m were compared with those for air-conducted 1 kHz tone bursts by magnetoencephalography. As a result, the growth of N1m amplitude for both stimuli saturated at the duration of 40 ms, which suggest that the temporal integration system of BCU is similar to that of audible sound. However, significant differences in the growth were observed below the saturation points. The results indicate a possibility that there are some differences in the central auditory system between BCU and audible sound.

Ultrasound can be heard by bone conduction in man. However, there has been no consensus about the perception mechanism of bone-conducted ultrasound (BCU). In the current study, to clarify the central auditory system of BCU, the effects of stimulus duration for 30 kHz BCU on N1m were compared with those for air-conducted 1 kHz tone bursts by magnetoencephalography. As a result, the growth of N1 nn amplitude for both stimuli saturated at the duration of 40 ms, which suggest that the temporal integration system of BCU is similar to that of audible sound. However, significant differences in the growth were observed below the saturation points. The results indicate a possibility that there are some differences in the central auditory system between BCU and audible sound. (C) 2002 Elsevier Science Ireland Ltd. All rights reserved.

The phenomenon physically occurring within the head for bone-conducted sound of various stimulation locations has been calculated using the finite-difference time-domain (FDTD) technique; three slightly different stimulation locations near the left mastoid were set for audible and ultrasonic frequency stimulation. Calculated sound fields at the plane including the cochleae showed considerably different characteristics at different stimulation frequencies. For audible frequency stimulation, their distribution negligibly differed for each stimulation location. On the contrary, for ultrasonic frequency stimulation, their distribution shifted considerably for each different stimulation location. These results indicated the characteristics of the shifting sound image perceived for bone-conducted ultrasound and the negligibly shifting sound image perceived for bone-conducted audible sound, from the slight changes in their stimulation locations.

The phenomenon physically occurring within the head for bone-conducted sound of various stimulation locations has been calculated using the finite-difference time-domain (FDTD) technique; three slightly different stimulation locations near the left mastoid were set for audible and ultrasonic frequency stimulation. Calculated sound fields at the plane including the cochleae showed considerably different characteristics at different stimulation frequencies. For audible frequency stimulation, their distribution negligibly differed for each stimulation location. On the contrary, for ultrasonic frequency stimulation, their distribution shifted considerably for each different stimulation location. These results indicated the characteristics of the shifting sound image perceived for bone-conducted ultrasound and the negligibly shifting sound image perceived for bone-conducted audible sound, from the slight changes in their stimulation locations.

Human cortical responses corresponding to the subjective preference for a flickering light of varying period were investigated. Paired-comparison tests were performed to examine the subjective preference for a flickering light, and MEG was recorded during presentations of the most preferred and less preferred flickering lights alternately. Results showed that the effective duration of the autocorrelation function, tau(e), which represents a repetitive feature of the MEG alpha waves, becomes longer during the preferred condition. This reveals that the brain repeats a similar rhythm under preferred conditions.

The quest to extract rock properties from seismic images has resulted in a growing interest in full waveform inversion and imaging. This study applies frequency-domain visco-acoustic waveform inversion and reverse-time imaging to synthetic and laboratory data. The laboratory data for a crosshole configuration is obtained with a two-axes computer-controlled scanning system. Broadband 200 kHz data is obtained using a piezofilm source and detector in a water tank with suspended acrylic bars. The velocity image produced by the waveform inversion has a higher resolution and more precisely determines the location of the acrylic bars compared to the traveltime tomography image. The Q imaging has poorer resolution and is less reliable than the velocity imaging. The reverse-time imaging of scattered waves can image the bars as well as the water surface.

Ultrasounds are perceived through bone-conduction by profoundly deaf as well as normal hearing subjects. A bone-conducted ultrasonic hearing aid (BCUHA) was developed for the profoundly deaf Parameters of BCUHA were determined by former psychoacoustic and neuromagnetic experiments, Using BCUHA. 53% of the Profoundly deaf subjects were able to perceive sounds. and 18 of them were able to recognize some words.

骨導で呈示された超音波(以下, 骨導超音波と呼ぶ)の聴覚心理学的特徴や知覚中枢部位などについてはこれまでに明らかにされてきたが, 末梢における聴覚系の関与などについては, いまだ不明な点が多く残されている.そこで本論文では, 頭部モデルを用いた実測及び数値シミュレーションを行い, 骨導音呈示時に生体中に形成される音場の計算結果などから, 骨導超音波知覚の末梢における関与について検討した.その結果, 刺激周波数が可聴域のときと超音波領域のときとで蝸牛付近に形成される音場に明らかな違いが見られた.また, 知覚音の聴覚心理学的特徴との関連も見られた.この結果は, 骨導超音波によって実現されると考えられている.重度の難聴者に対する補聴システムの開発に有効な設計指針を与えるものである.

The aim of this study is to identify the relationship between subjective preference and the human brain responses to visual motion under changing its period. First, preference judgments using the paired-comparison method for sinusoidal movements of a single circular target in horizontal direction were performed. Then, MEG data were recorded during presentations of the most preferred and the least preferred moving stimuli. From the initial delay range of ACF of the alpha wave, the effective duration (tau(c)) was analyzed. Results show that the stimulus with most preferred periods has a greater value of tau(c) than that with least preferred periods at the occipital area, especially in the left hemisphere.

The cortical site which processes information on whole-body linear displacement is unknown. In this study, neuromagnetic responses to a visually-induced linear vection were recorded in 5 healthy, right-handed, adult subjects using a 122-channel whole cortex neuromagnetometer. We presented expanding rectangles on the screen which came into view one after another and accelerated in expanding speed at random cycle, giving the subjects the sensation of linear self motion (linear vection) through an illusory tunnel with occasional acceleration. Clear responses of magnetic fields related to the accelerative event were obtained in both hemispheres around the parietal and temporal regions. The dipole sources of the component were estimated in the cortex around the superior temporal sulcus, insula and medial superior temporal area. Some parts of these regions may have been comprised in the vestibular cortex, suggesting that it processes the sensation of linear self motion and plays an important role in space perception.

The aim of this study is to identify the relationship between subjective preference and the human brain responses to visual motion under changing its period. First, preference judgments using the paired-comparison method for sinusoidal movements of a single circular target in horizontal direction were performed. Then, MEG data were recorded during presentations of the most preferred and the least preferred moving stimuli. From the initial delay range of ACF of the alpha wave, the effective duration (τe) was analyzed. Results show that the stimulus with most preferred periods has a greater value of τe than that with least preferred periods at the occipital area, especially in the left hemisphere.

The cortical site which processes information on whole-body linear displacement is unknown. In this study, neuromagnetic responses to a visually-induced linear vection were recorded in 5 healthy, right-handed, adult subjects using a 122-channel whole cortex neuromagnetometer. We presented expanding rectangles on the screen which came into view one after another and accelerated in expanding speed at random cycle, giving the subjects the sensation of linear self motion (linear vection) through an illusory tunnel with occasional acceleration. Clear responses of magnetic fields related to the accelerative event were obtained in both hemispheres around the parietal and temporal regions. The dipole sources of the component were estimated in the cortex around the superior temporal sulcus, insula and medial superior temporal area. Some parts of these regions may have been comprised in the vestibular cortex, suggesting that it processes the sensation of linear self motion and plays an important role in space perception.

To investigate the characteristics of visual short-term memory in humans, brain magnetic fields evoked during a delayed paired comparison task were recorded using a whole-head neuromagnetometer. The visual stimulus consisted of a circle with different colors in each quadrant. In the memory condition, subjects reacted with the index finger, when the first stimulus (Sample) was identical in color configuration to the second stimulus (Test), and with the middle finger when they differed. For the control condition, the Subjects ignored the Sample, and moved the index or middle finger alternately in response to the Test. Extremely low frequency components of brain magnetic fields were observed 500 ms after the Sample onset in the temporal and/or the occipital region in the memory condition, but not in the control condition, Sources for the low frequency components were localized in the inferior part of the occipital lobe, in the vicinity of the supramarginal gyrus and the angular gyrus, and the inferior frontal gyrus, The results suggest that the activities in the inferior part of the occipital lobe controls the storage process of shortterm visual memory.

N1m is an auditory evoked brain magnetic field with a magnitude of 100 fT order observed over the auditory cortex, 100 ms after the onset of auditory stimuli. The N1m is often used as a landmark of functional localization in the cortex. However, the mechanism of the N1m has not yet been clarified. The N1m peak amplitude and latency are dependent on the specifics of the stimulus; duration, intensity, and sequence of stimuli. In this study, we examined the dependency of the N1m peak amplitude and latency on the stimulus duration and frequency. Trains of 0.2 ms clicks were used for auditory stimuli by changing the number of clicks and the click interval. Auditory brain magnetic responses evoked by the click trains were recorded from seven human adult subjects by a dc superconducting quantum interference device magnetometer. In the results of this study, the N1m amplitudes significantly increased as the stimulus duration increased and the amplitudes leveled when the stimulus duration reached 32 ms. The amplitudes produced by the trains with the same number of clicks showed greater values for 4-ms-interval trains. The N1m latencies significantly decreased as the stimulus duration increased and leveled at 32 ms. It is concluded that all clicks received within 32 ms were integrated and that this integration mechanism is dependent upon the click interval. Increased synchrony of neuronal cells at the cortical level can explain this integration mechanism. (C) 1999 American Institute of Physics. [S0021-8979(99)42908-1].

Auditory evoked magnetic fields were measured to investigate the reaction of the auditory cortex to a train of sounds. Auditory stimuli, 1000-Hz tone bursts (percentage of appearance: 90%), and 2000-Hz tone bursts (percentage of appearance: 10%) were presented sequentially. The stimuli were presented with the stilmulus onset asynchronies (SOAs) within one trial fixed at either 380 ms, 480 ms, 580 ms, 1080 ms, 1580 ms, or 2080 ms. Nlm amplitudes of 2000-Hz tone bursts, 1000-Hz tone bursts 1 position before, 1 position after, 2 positions after, and 3 positions after the 2000-Hz tone bursts, were investigated. As a result, (1) the amplitudes of Nlm of 2000-Hz tone bursts were significantly larger than those of 1000-Hz tone bursts except for SOA of 2080 ms, (2) the amplitudes of Nlm increased as the SOA increased except for 380 ms and 480 ms, (3) the Nlm amplitude ratios of 1000-Hz tone bursts to 2000-Hz tone bursts increased as the SOA increased, (4) no significant effects of serial position on Nlm amplitude of 1000-Hz tone bursts were observed.

To investigate the characteristics of short-term memory in the human brain, magnetoencephalograms (MEGs) evoked by a delayed paired comparison task were recorded. A visual stimulus, a circle with a different color in each quadrant, was presented. In the memory task, subjects were requested to move the index finger when the second stimulus (TEST) was identical to the first stimulus (SAMPLE), and the middle finger when it was not identical. In the control task, the subjects were instructed not to pay attention to the SAMPLE, and to move the index and middle fingers alternately. A slow magnetic activity was observed for between 900 and 1500 ms during the memory task in the occipital region of all subjects. Sources for this slow activity were localized in either the visual cortex and/or the posterior temporal region. The results suggest that these areas may be responsible for retaining the stimulus in visual short-term memory.

In order to investigate the characteristics of human visual short-term memory, evoked brain magnetic fields during a delayed paired comparison task were recorded with a whole-head neuromagnetometer. A visual stimulus was a circle with different colors (red, blue, green, and orange) in each quadrant. The interstimulus interval between the first stimulus (sample; duration 50 ms) and the second stimulus (test; duration 100 ms) was constant at 3.0 s. Two experiments were performed with different tasks; a memory task and a control task. In the memory task, subjects reacted with the index finner, when the test was identical to the sample in color configuration, and with the middle finger when they were not identical. In the control task, the subjects paid no attention to the sample, and moved the index or middle finger alternately in response to the test. Extremely low frequency components of the brain magnetic fields were observed between 900 and 1,500 ms in the occipital region in the memory task but not in the control task. Sources for the low-frequency components were localized in the visual cortex and/or the posterior temporal region. The results suggest that these areas may be engaged in retaining the stimulus in the visual shortterm memory.