FUNCTIONAL LOCALIZATION OF BILATERAL AUDITORY CORTICES USING AN MRI-LINKED WHOLE HEAD MAGNETOENCEPHALOGRAPHY (MEG) SYSTEM

In 20 healthy subjects, auditory evoked magnetic fields were measured over the entire head, using a helmet-shaped 66-channel MEG system link ed to MRI. When the left or right ear was stimulated by 60 msec 2 kHz tones, the prominent 100 msec response (N100m) appeared significantly earlier in the contralateral hemisphere than in the ipsilateral one. I n 16 cases, the N100m dipolar field patterns were clear in both hemisp heres, overlapping each other across the midline. The N100m sources we re estimated using a 2-dipole model in a spherical conducting medium w ith the size and location of the sphere determined individually accord ing to the MRI images. No differences were found between the contralat eral and ipsilateral N100m dipole positions in one hemisphere. When su perimposed on MRI, the N100m dipoles were located precisely on the upp er surface of bilateral temporal lobes with a standard deviation of 2. 2 mm in the superior-inferior direction. In 16 right handed males, the right hemispheric N100m dipoles were 6 mm anterior to the left hemisp heric dipoles. The whole head MEG is suitable to see small but signifi cant differences of bilateral cerebral function, with exceptionally hi gh spatial resolution, confirmed by the MRI-linked system.