Origin of Somatosensory Evoked Scalp Responses in Man

ECAUSE of similarities between somatosensory evoked potentials recorded from the scalp and those arising directly from the brain (animal or man), early components of the scalp response have been thought to arise from the primary sensory area, late ones to originate more diffusely in associated areas or midline cortex at the vertex. 2,5-1~ However, whether these assumptions are correct, whether there is an ipsilateral as well as a contralateral cortical contribution, or whether the entire scalp response derives only from the contralateral primary sensory area, are questions which remain unanswered and with which this paper will deal. The study centers on a group of patients who had selective cortical ablations for epilepsy. For our purpose they afforded a unique opportunity in that some required excision of the somatosensory hand area, either alone, or in addition to other tissue, thus enabling us to compare the effects of such ablations, on the scalp response, with that of other cerebral excisions. Also, during surgery multiple cortical recordings were made from the entire exposed field to identify all cortical areas that are capable of generating a somatosensory response. Comparison of these cortical responses with scalp potentials that were elicited simultaneously provided additional important information. Methods