The second sensory area in humans: evoked potential and electrical stimulation studies.

A patient with intractable seizures originating from a right frontal focus was evaluated for surgical treatment. This evaluation was carried out using a chronically implanted array of 96 stainless steel electrodes 1 cm apart and covering the perirolandic and frontal areas. Somatosensory evoked potentials and electrical stimulation of the subdural electrodes localized the primary sensory hand area. Evoked potentials of identical waveform but of lower amplitude and 2.4 ms longer latency were recorded in the inferior frontal gyrus immediately anterior to the face area of the motor strip. Electrical stimulation of that area elicited: (1) a "paralyzing" feeling in the left arm and face; (2) inhibition of rapid alternating movements of left fingers, left hand, and tongue; (3) inability to maintain a strong voluntary muscle contraction of the left hand or tongue; and (4) speech arrest. This appears to be the first report of a secondary sensory area in humans demonstrated by both electrical stimulation and evoked potential studies. Electrical stimulation showed that the secondary sensory area overlapped an area of complex motor control, suggesting that the secondary sensory area provides direct sensory feedback information for appropriate motor integration.