\"DOMINANCY\" IN THE SECOND SOMATOSENSORY AREA REVEALED BY MAGNE TOENCEPHALOGRAPHY

The second somatosensory area (SlI) has been studied both by electrical evoked potentials24 and magnetoencephalography (MEG)5 Magnetic evoked fields of contralateral primary somatosensory and ipsilateral second somatosensory cortices of 12 normal subjects were recorded in response to median nerve electrical stimulation by means of a single magnetometer. We detected. in addition to the usual contralateral SI responses, ipsilateral second somatosensory evoked fields (sSEF's) in 4 subjects at a latency of 35-45 ms over the dominant hemisphere for handedness, i.e., right hemisphere in left handed subjects and vice versa. On no occasion were ipsilateral somatosensory fields recorded from the non-dominant hemisphere whichever median nerve was stimulated. There were no consistent responses at 90-150 ms. We concluded that the ipsilateral responses were from the SlI area. Our observations indicate the following: a) despite the known bilateral representation of SII, there is hemispheric dominance, b) we could not confirm the long latency (>95 ms) signals reported by Hari et al. in any of our subjects.5.6