Bilateral somatosensory evoked potentials in four patients with long-standing surgical hemispherectomy

Four patients were studied electrophysiologically 8 to 24 years after surgical removal of one cerebral hemisphere without damage to the striatum or diencephalon. Somatosensory evoked potentials (SEPs) to electrical stimulation of the median nerve on the left or right side were averaged and mapped out over the scalp. Stimulation on the side opposite to the missing hemisphere evoked brief P9 and P14 farfields and a slow N18 negative potential of 15- to 25-msec duration bilaterally. No additional focal response was detected over the remaining (ipsilateral) hemisphere for 60 msec after the stimulus. Because long-standing hemispherectomy entails massive retrograde degeneration of thalamocortical neurons, the preserved P14 and N18 responses must reflect neural activities generated below the thalamus that are volume conducted to the scalp bilaterally. The data clarify several current issues in the evaluation of SEP components.