Neuromagnetic study of the auditory responses in right and left hemispheres of the human brain evoked by pure tones and speech sounds.

Neuromagnetic responses in the human auditory cortex evoked by various burst stimuli of pure tones and monosyllable speech sounds were measured separately from two hemispheres. Both the pure tones and speech sounds elicited clear responses with two main peaks. The field patterns over the scalp at the peak latencies indicated a single current dipole as an equivalent field generator. The depth of the current dipoles computed from the mapped field data was deeper from the scalp for a tone stimulus of higher frequency, which confirms the tonotopic organization in the auditory cortex. A difference was found in the dipole locations in the horizontal plane for the speech stimuli of a vowel and a consonant-vowel /a/ and a consonant-vowel /ka/. It suggests the sensitivity of the magnetic responses to the acoustic structure of the speech sound.