IPSILATERAL MEDIAN SOMATOSENSORY-EVOKED POTENTIALS RECORDED FROM HUMAN SOMATOSENSORY CORTEX

Somatosensory evoked potentials (SEP) to ipsilateral and contralateral median nerve stimulations were recorded from subdural electrode grids over the perirolandic areas in 41 patients with medically refractory focal epilepsies who underwent evaluation for epilepsy surgery, All pa tients showed clearly defined, high amplitude contralateral median SEP s. In addition, four patients showed ipsilateral SEPs. Compared with t he contralateral SEPs, ipsilateral SEPs were very localized, had a dif ferent spatial distribution, were of considerably lower amplitude, had a longer latency (1.2-17.S ms), did not show an initial negativity, a nd were markedly attenuated during sleep. Stimulation of the subdural electrodes overlying the sensory hand area was associated with contral ateral hand paresthesias, but no ipsilateral hand paresthesias occurre d. It was concluded that subdurally recorded cortical SEPs to ipsilate ral stimulation of the median nerve (M) reflect unconscious sensory in put from the hand possibly serving fast bimanual hand control, The ana tomical pathway of these ipsilateral short-latency MSEPs is not yet kn own. Transcallosal transmission seems unlikely because of the short de lay between the ipsilateral and contralateral responses in selected ca ses. The infrequent occurrence of ipsilateral subdurally recorded SEPs and their low amplitude and limited distribution suggest that they co ntribute very little to the short-latency ipsilateral median SEPs reco rded on the scalp. (C) 1997 Elsevier Science Ireland Ltd.