Yosuke Okamoto

Intelligibility of bone-conducted ultrasonic speech

Ultrasound can be perceived through bone conduction by the profoundly deaf as well as by normal-hearing subjects. Moreover, speech signals modulated onto ultrasound can be detected through bone conduction. This study explored how well listeners can understand ultrasonic speech and the confusion patterns to evaluate and improve bone-conducted ultrasonic hearing. The intelligibility of Japanese words classified by familiarity and Japanese monosyllables with bone-conducted ultrasound was investigated. Results showed that the intelligibility of familiar words was higher than that of unfamiliar words. Further, the results of a monosyllable intelligibility test with bone-conducted ultrasound and those of a test with air-conducted sound showed a similar pattern of speech recognition with regard to the errors made. The relationship between speech intelligibility and sound level showed that the increase in the intelligibility of bone-conducted ultrasonic speech did not exceed the increase in the intelligibility of air-conducted speech as the sound level rose.

Effects of daytime light exposure on cognitive brain activity as measured by the ERP P300.

Exposure to light modulates not only human alertness but also cognitive functions. The present study examined the temporal dynamics of the effects of light exposure on cortical activity related to cognitive processes. Event-related potentials (ERPs) were measured while participants performed an auditory oddball task during exposure to short-, medium- or long-wavelength light or darkness. Experiments were conducted in the daytime. After a 10-min period of darkness, one of the three lights was presented for 28 min. In the control condition, darkness was maintained for the entire session. The ERP component observed approximately 300 ms after the onset of the target stimulus (P300) was analyzed. The amplitude of P300 was larger after 5-20 min of exposure to short-wavelength light than at equivalent time points in the darkness. No differences were observed in the amplitude of P300 between the medium- or long-wavelength light condition and darkness at any time point. These results suggest that the amount of attentional resource allocated to the oddball task was increased by daytime exposure to short-wavelength light, and that following approximately 5 min of exposure the impact of light on cortical activity related to cognitive processes was able to be detected.

Autocorrelation analyses of magnetoencephalographic alpha waves in relation to subjective preference for a flickering light.

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, &tgr;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.

Effects of light wavelength on MEG ERD/ERS during a working memory task

We investigated the effects of light wavelengths on cortical oscillatory activity associated with working memory processes. Cortical activity responses were measured using magnetoencephalography (MEG) while participants performed an auditory Sternberg memory task during exposure to light of different wavelength. Each trial of the memory task consisted of four words presented as a memory set and one word presented as a probe. All words were presented audibly. Participants were instructed to indicate whether the probe word was or was not presented within the memory set. A total of 90 trials were conducted under the light exposure. Event-related synchronization (ERS) and event-related desynchronization responses in the alpha frequency range during the task were analyzed. Results showed that, during memory encoding, ERS responses were significantly greater in the short-wavelength (blue) light condition than in the middle-wavelength (green) light condition, approximately 20-30min after the onset of light exposure. Behavioral performance was very high throughout the experiment and there was no difference between the light conditions. Although the light effects were not observed in behavior, the result of ERS suggests that 20-30min of exposure to blue light enhances cortical activity related to active memory maintenance and/or attention to auditory stimuli.

Effects of the Degree of Fluctuation on Subjective Preference for a 1 Hz Flickering Light

Humans are believed to have a preferred amount of stimulus variation in their perceptual environment. Here, paired comparison tests were conducted to examine whether the fluctuation of a flickering light improves subjective preference. Sine-wave and bandpass noise acted as the light source. We have previously shown that the preferred temporal frequency of a flickering light without any fluctuation is approximately 1 Hz (Soeta et al 2002 Journal of the Optical Society of America A 19 289 – 294). This was used as the center frequency of the light source. The bandwidth was set at 1, 2, 4, 8, and 16 Hz, to control the amplitude of the first peak of the autocorrelation function, ø1 Results show that the preferred ø1 of a flickering light is 0.46.

Comparison of MEG responses to the sinusoidal flicker and the envelope of amplitude-modulated flicker.

For a flickering light with two or more frequency components, the temporal envelope of the flickering light is perceived, although there is no frequency component at the frequency of the envelope. To address the processing of the temporal envelope, we compared cortical responses to the envelope of amplitude-modulated (AM) flickering lights with responses to sinusoidal flickering lights. We measured magnetoencephalographic (MEG) responses to AM and sinusoidal flickering lights while changing the frequency of the temporal envelope of AM flickering light and the stimulus frequency of sinusoidal flickering light. The result showed that the strength of the MEG response to sinusoidal flickering light increased with stimulus frequency in the low frequency range. This probably resulted from the band-pass filtering in the visual system. However, such an increase in the low frequency range was not observed for the envelope of AM flickering light. The absence of the increase in MEG responses suggests that the envelope of AM flickering light is not subject to band-pass filtering in the visual system. It is therefore possible that the envelope is represented after the band-pass filtering stage.

Image Color Conversion by Illumination

We developed a software program to imitate the color of an object within the range of sRGB on a PC display. The obtained colors were converted from the picture information taken under a white light source, such as D65. Other, arbitrary light sources such as natural sunlight or standard illuminants, could be used in addition to the test illumination. Judging from the sample of the converted pictures, the software functions quite well.

Effects of daytime exposures to short- and middle-wavelength lights on cortical activity during a cognitive task

This study aimed to assess the effects of light wavelength on human cortical activity during a cognitive task, by observing event-related potentials (ERPs). We measured ERPs when subjects perform an auditory oddball task under daytime exposures to short- and middle-wavelength lights. In the oddball task, a standard tone of 1 kHz and a target tone of 2 kHz were used. Subjects were instructed to press a button as soon as they heard the target tone. The ERP component that occurs approximately 300 ms after a stimulus onset (P300) was analyzed. The results showed that amplitude of P300 in the short-wavelength light condition was significantly larger than that in the middle-wavelength light condition approximately 10-20 min after the start of light exposure. It has been suggested that the amplitude of P300 reflects the amount of attentional resources for a given task. Therefore, the results obtained suggest that amount of attentional resource allocated to the oddball task is increased during exposure to the short-wavelength light in the daytime.

Effects of adaptation on the temporal envelope of amplitude-modulated flickering light.

This study sought to investigate the way in which the temporal envelope of amplitude-modulated (AM) flickering light is processed in the visual system. To this end, we measured the effects of adaptation on a low-frequency (2 Hz) envelope of AM flickering light with a high carrier frequency (16 Hz). The results showed that sensitivity to the envelope of the AM flickering light was reduced by adaptation to the low temporal frequency, although the AM flickering light had a frequency component at the carrier frequency but not at the frequency corresponding to the envelope. These results suggest that the low-frequency temporal envelope, composed exclusively of high-frequency (first order) Fourier energy, is encoded by a low-frequency selective channel.

Effects of Adaptation on Visual Sensitivity and MEG Responses to the Envelope of AM Flicker

For a sinusoidal flickering light amplitude- modulated (AM) sinusoidally, flicker with a periodicity of a temporal envelope (modulation-frequency) of the light is perceived even though there is no such frequency component. However, the mechanisms underlying the perception are still unclear. In human visual system, it is known that there are two temporal channels with a lower and a higher peak frequency. Therefore, we investigated whether the two temporal channels affected the perception of the envelope of the AM flickering light. Visual sensitivity and magnetoencephalographic (MEG) responses to the envelope periodicity were measured when the sensitivity of each of the temporal channel was reduced by adaptation. Sinusoidal flickering lights with a low (2 Hz) and a high (16 Hz) temporal frequency were used as adapting stimuli. The sinusoidal flickering lights of 2 and 16 Hz and an AM flickering light with a modulation-frequency of 2 Hz and a carrier-frequency of 16 Hz were presented as test stimuli. The results showed that the sensitivity and MEG responses to the sinusoidal flickering light decreased more when the sensitivity of the temporal channel tuned for the test stimulus frequency was reduced than when that of the other temporal channel was reduced. On the other hand, the sensitivity and MEG responses to the envelope periodicity decreased more when the sensitivity to the frequency corresponding to the envelope periodicity was reduced than when that to the carrier frequency was reduced, even though a frequency component corresponding to the envelope periodicity did not exist in the stimulus. The results indicated that the envelope component of AM flickering light was affected by the low-frequency channel and suggest the possibility that the envelope component arises before or while the AM flickering signal passes the low-frequency channel.