Ipsilateral somatosensory responses in humans: the tonic activity of SII and posterior insular cortex

In the present study, we mapped the spatio-temporal dynamics of cortical responses to ipsilateral median nerve stimulation using intracerebral recordings (stereo-EEG) in 38 drug-resistant epileptic patients. Furthermore, we compared the pattern of responsiveness obtained in the same leads across ipsilateral and contralateral stimulations. Ipsilateral responses were found mostly confined to SII and posterior insula, while no activity was found in ipsilateral SI. By examining the temporal profiles of activation, ipsilateral SII showed a prominent tonic pattern, while contralateral SII exhibited both phasic and tonic responses. Beyond the localization of the active cortical nodes, these data contributed to identify the cortico-cortical connections carrying the somatosensory information to the ipsilateral hemisphere, with a major role of transcallosal projections from contralateral SII. In light of previous literature and of its localization, the functional role possibly covered by long lasting discharge in SII and insular cortex is also discussed. Overall, the presence of tonic activities was neglected so far due to the impossibility to identify deep sources along with a resolved description of their time course. The use of stereo-EEG, instead, allows one to achieve a four-dimensional characterization, complementing the classical view about the somatosensory system organization.