Timing and spatial distribution of somatosensory responses recorded in the upper bank of the Sylvian fissure (SII area) in humans

We studied responses of the parieto-frontal opercular cortex to electric stimuli, as recorded by intra-cortical electrodes during stereotactic EEG presurgical assessment of patients with drug-resistant temporal lobe epilepsy. After electrical stimulation of the median nerve at the wrist, we consistently recorded a negative-positive biphasic response peaking at 60 ms (N60) and 90 ms (P90) post-stimulus in the upper bank of the sylvian fissure contralateral to stimulation. Talairach stereotactic coordinates of the electrode contacts recording these responses covered the pre- and post-rolandic part of the upper bank of the sylvian fissure (25 < x < 55 mm; -27 < y < +13 mm; 0 < z < 21 mm), corresponding to the accepted localization of SII area in man. The sources of these responses were deeply situated in the cortex of the upper bank of the sylvian fissure at similar to 40 mm from the midline sagittal plane, so that some of them could be located in the insular cortex. Moreover this study suggests the existence of dipolar SII sources radial to the scalp surface, which are overlooked in magnetic recordings. Somatosensory evoked potentials (SEPs) recorded in SII are delayed by similar to 40 ms as compared with SEPs generated in the primary somatosensory cortex. This long delay between SI and SII responses is not fully explained though it is coherent with the timing of activation issued from MEG source modeling data.