Biomedical subjects

Hidehiko Okamoto

Publications and source records attributed to Hidehiko Okamoto.

4 recordsLinked to original sources

PubMed2005

Serial and parallel processing in the human auditory cortex: a magnetoencephalographic study.

Although anatomical, histochemical and electrophysiological findings in both animals and humans have suggested a parallel and serial mode of auditory processing, precise activation timings of each cortical area are not well known, especially in humans. We investigated the timing of arrival of signals to multiple cortical areas using magnetoencephalography in humans. Following click stimuli applied to the left ear, activations were found in six cortical areas in the right hemisphere: the posteromedial part of Heschl's gyrus (HG) corresponding to the primary auditory cortex (PAC), the anterolateral part of the HG region on or posterior to the transverse sulcus, the posterior parietal cortex (PPC), posterior and anterior parts of the superior temporal gyrus (STG), and the planum temporale (PT). The mean onset latencies of each cortical activity were 17.1, 21.2, 25.3, 26.2, 30.9 and 47.6 ms respectively. These results suggested a serial model of auditory processing along the medio-lateral axis of the supratemporal plane and, in addition, implied the existence of several parallel streams running postero-superiorly (from the PAC to the belt region and then to the posterior STG, PPC or PT) and anteriorly (PAC-belt-anterior STG).

Adult↗

PubMed2003

Magnetoencephalographic study of the cortical activity elicited by human voice.

In an attempt to identify voice-specific neural activities in auditory cortex in humans, we recorded cortical magnetic responses. Volunteers were instructed to listen to vocal and instrumental sounds matched in fundamental-frequency, duration, temporal envelope and average root mean square power. The stimuli were sounds produced by four singers and four musical instruments at each of two fundamental frequencies: 220 Hz (musical note A3) and 261.9 Hz (C3). Two components of the evoked responses were analyzed, one at approximately 100 ms (N1m) and the other 400 ms after the stimulus onset (sustained field, SF). The source locations of equivalent current dipoles for both components were estimated around the Heschl's gyrus in both hemispheres. Compared with the instrumental sound, the source strength of the SF component for the voice was significantly larger.

Acoustic Stimulation↗

PubMed2003

Current source density distribution of sleep spindles in humans as found by synthetic aperture magnetometry.

Previous magnetoencephalography (MEG) studies of sleep spindles have identified a complex and widespread distribution in parietal and frontal cortices by fitting a dipolar model to the data. In this study, we used a whole-head 151-channel MEG system with simultaneous electroencephalography (EEG) recording in eight normal subjects. All subjects fell asleep during stage 2 and 3; EEG spindles have been observed in all cases. The current source density distribution of sleep spindles in the 10-15 Hz frequency band was localized by means of synthetic aperture magnetometry, and statistically tested using a permutation analysis. Source locations of the sleep spindles were found primarily in the frontal cortex, including dorsolateral and medial prefrontal areas, as well as the parietal cortex, including the vicinity of the primary sensorimotor areas. These results suggest that sleep spindles are similar in frequency content and spatial location to mu rhythm, and that these two oscillatory activities might also have a common neural basis and physiological meaning.

Adult↗

PubMed2003

Tonotopic representation of missing fundamental complex sounds in the human auditory cortex.

The N1m component of the auditory evoked magnetic field in response to tones and complex sounds was examined in order to clarify whether the tonotopic representation in the human secondary auditory cortex is based on perceived pitch or the physical frequency spectrum of the sound. The investigated stimulus parameters were the fundamental frequencies (F0 = 250, 500 and 1000 Hz), the spectral composition of the higher harmonics of the missing fundamental sounds (2nd to 5th, 6th to 9th and 10th to 13th harmonic) and the frequencies of pure tones corresponding to F0 and to the lowest component of each complex sound. Tonotopic gradients showed that high frequencies were more medially located than low frequencies for the pure tones and for the centre frequency of the complex tones. Furthermore, in the superior-inferior direction, the tonotopic gradients were different between pure tones and complex sounds. The results were interpreted as reflecting different processing in the auditory cortex for pure tones and complex sounds. This hypothesis was supported by the result of evoked responses to complex sounds having longer latencies. A more pronounced tonotopic representation in the right hemisphere gave evidence for right hemispheric dominance in spectral processing.

Acoustic Stimulation↗