We have selected a subgroup of 11 out of 168 patients with a pathological lesion in or adjacent to the primary sensorimotor cortex assessed by median nerve somatosensory evoked potentials (SEPs). In this subgroup a simplification of waveform with predominant deep positivity was recorded on the lesion side. Similar findings have been previously reported in patients in deep coma. A mechanism similar to the “killed end potential” and subcortical origin of deep positivity has been suggested. Possible mechanisms generating a large positive potential in the terms of volume conduction principles are discussed.

        Key words: somatosensory evoked potentials, positive wave, parietal tumors, central cortex