SOURCE ANALYSIS OF MAGNETIC-FIELD RESPONSES FROM THE HUMAN AUDITORY-CORTEX ELICITED BY SHORT SPEECH SOUNDS

We made a detailed source analysis of the magnetic field responses tha t were elicited in the human brain by different monosyllabic speech so unds, including vowel, plosive, fricative, and nasal speech. Recording s of the magnetic field responses from a lateral area of the left hemi sphere of human subjects were made using a multichannel SQUID magnetom eter, having 37 field-sensing coils. A single source of the equivalent current dipole of the field was estimated from the spatial distributi on of the evoked responses. The estimated sources of an Nlm wave occur ring at about 100 ms after the stimulus onset of different monosyllabl es were located close to each other within a 10-mm-sided cube in the t hree-dimensional space of the brain. Those sources registered on the m agnetic resonance images indicated a restricted area in the auditory c ortex, including Heschl's gyri in the superior temporal plane. In the spatiotemporal domain the sources exhibited apparent movements, among which anterior shift with latency increase on the anteroposterior axis and inferior shift on the inferosuperior axis were common in the resp onses to all monosyllables. However, selective movements that depended on the type of consonants were observed on the mediolateral axis; the sources of plosive and fricative responses shifted laterally with lat ency increase, but the source of the vowel response shifted medially. These spatiotemporal movements of the sources are discussed in terms o f dynamic excitation of the cortical neurons in multiple areas of the human auditory cortex.