Trigeminal evoked potentials in somatosensory cortex of the Macaca mulatta.

Somatosensory evoked potentials (SEPs) obtained in response to stimulation of the median nerve, posterior tibial nerve, lips, tongue, palate, and pharynx were recorded in four monkeys (Macaca mulatta) under light barbiturate anesthesia. In agreement with the results of SEP recordings in humans and single-unit recordings in monkeys, there is a medial-to-lateral representation in somatosensory cortex of the hand, lips, and tongue. There is a discontinuity in the representation of the upper mouth: the tongue representation is interposed between a medial region (near the lip representation), usually representing the lateral hard palate and gum, and a lateral region (near the lateral sulcus), usually representing the central hard palate. For all types of trigeminal stimulation, root mean square (RMS) voltage maps show maxima over somatosensory cortex. Laminar recordings demonstrated that trigeminal SEPs recorded from the cortical surface are generated in somatosensory cortex in areas 1, 2, and 3b. Median nerve and posterior tibial nerve stimulation did not evoke SEPs in the surface cortex near the lateral sulcus, suggesting that the most lateral portion of the postcentral gyrus is not a part of the second somatosensory area (SII). This lateral region may contain the representation of posterior intraoral structures, but it was not possible to confirm this assumption by stimulation of the soft palate or pharynx.