Neiuromagnetic study on speech sound processing in human auditory cortex

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We have been studied the neural activities in the human auditory cortex concerning with the speech sound processing by measuring neuromagnetic fields. Obtained results are summarized as follows.1. Background noises in the auditory evoked magnetic field responses recorded with a 37 channel SQUID magnetometer have been studied. The background noise was found to have a main contribution from the spontaneous fields, which originate in the neural activities of the brain. The spontaneous fields had strong spatial correlation across the recording sites even after averaging over 100 epochs. Furthermore, the signal to noise ratio, which is needed for accurate source localization, was determined from the experiments.2. The relationship between tinnitus and auditory neural activities was studied based on the neuromagnetic fields. It was found that either Nlm or Plm component increases due to the suppression of tinnitus by electrical stimulation to cochlea under some circumstances.3. Neuromagnetic fields evoked by monosyllable speech sounds, i.e. /a/, /ka/, /ha/ and /na/, have been measured using a 37 channel SQUID magnetometer. Parameters of a dipole current source have been estimated using several sets of field data recorded in different measurement runs. It was found from the results that statio-temporal movements of the sources depend on the subjects as well as stimuli. However, the sources of N1m components evoked by /a/ and /na/ commonly move either to anterior, medial or posterior direction in all subjects.