Topography and cortical generators of somatosensory potentials evoked by median nerve stimulation in the cat

Epidural and cortical mapping of somatosensory evoked activity after median nerve stimulation was performed under barbiturate anaesthesia in 6 cats. Depth profiles were made to confirm the site of the cortical generators. The area studied revealed two cortical generators in SI and one in SII. In all cases polarity changes in intracortical tracks were demonstrated. The peak latency of all these generators was 12 msec. In SI a P8 was also a consistent finding in epidural, epicortical and intracortical measurements. No evidence, however, could be obtained for a cortical origin of the P8. The most rostral generator of the P12 was localized in the posterior sigmoid gyrus (area 3a). The N12 originates from the dorso-medial bank of the coronal sulcus in SI (area 2). Histological evidence for these projections was obtained with use of electrode markation. Within the second somatosensory area one source was active; the P12 originated just lateral to the anterior aspect of the suprasylvian sulcus in the anterior ectosylvian gyrus.