ACTIVATION OF A DISTRIBUTED SOMATOSENSORY CORTICAL NETWORK IN THE HUMAN BRAIN - A DIPOLE MODELING STUDY OF MAGNETIC-FIELDS EVOKED BY MEDIANNERVE-STIMULATION .1. LOCATION AND ACTIVATION TIMING OF SEF SOURCES

Cortical areas responsive to somatosensory inputs were assessed by rec ording somatosensory evoked magnetic fields (SEF) to electrical stimul ation of the left median nerve at wrist, using a 122-SQUID neuromagnet ometer in various conditions of stimulus rate, attentional demand and detection task. Source modelling combined with magnetic resonance imag ing (MRI) allowed localisation of six SEF sources on the outer aspect of the hemispheres located respectively: (1) in the posterior bank of the rolandic fissure (area SI), the upper bank of the sylvian fissure (parietal opercular area SII) and the banks of the intraparietal fissu re contralateral to stimulation, (2) in the SII area ipsilateral to st imulation and (3) in the mid-frontal or inferior frontal gyri on both sides. All source areas were found to be simultaneously active at 70-1 40 ms after the stimulus, the SI source was the only one active alread y at 20-60 ms. The observed activation timing suggests that somatosens ory input from SI is processed to higher-order areas through serial fe edforward projections. However the long-lasting activations of all sou rces and their overlap in time is also compatible with a top-down cont rol mediated via backward projections. (C) 1997 Elsevier Science Irela nd Ltd.