1. The anatomic generators of human median nerve somatosensory evoked potentials (SEPs) in the 40 to 250-ms latency range were investigated in 54 patients by means of cortical-surface and transcortical recordings obtained during neurosurgery. 2. Contralateral stimulation evoked three groups of SEPs recorded from the hand representation area of sensorimotor cortex: P45-N80-P180, recorded anterior to the central sulcus (CS) and maximal on the precentral gyrus; N45-P80-N180, recorded posterior to the CS and maximal on the postcentral gyrus; and P50-N90-P190, recorded near and on either side of the CS. 3. P45-N80-P180 inverted in polarity to N45-P80-N180 across the CS but was similar in polarity from the cortical surface and white matter in transcortical recordings. These spatial distributions were similar to those of the short-latency P20-N30 and N20-P30 potentials described in the preceding paper, suggesting that these long-latency potentials are generated in area 3b of somatosensory cortex. 4. P50-N90-P190 was largest over the anterior one-half of somatosensory cortex and did not show polarity inversion across the CS. This spatial distribution was similar to that of the short-latency P25-N35 potentials described in the preceding paper and, together with our and Goldring et al. 1970; Stohr and Goldring 1969 transcortical recordings, suggest that these long-latency potentials are generated in area 1 of somatosensory cortex. 5. SEPs of apparently local origin were recorded from several regions of sensorimotor cortex to stimulation of the ipsilateral median nerve. Surface and transcortical recordings suggest that the ipsilateral potentials are generated not in area 3b, but rather in other regions of sensorimotor cortex perhaps including areas 4, 1, 2, and 7. This spatial distribution suggests that the ipsilateral potentials are generated by transcallosal input from the contralateral hemisphere. 6. Recordings from the periSylvian region were characterized by P100 and N100, recorded above and below the Sylvian sulcus (SS) respectively. This distribution suggests a tangential generator located in the upper wall of the SS in the second somatosensory area (SII). In addition, N125 and P200, recorded near and on either side of the SS, suggest a radial generator in a portion of SII located in surface cortex above the SS. 7. In comparison with the short-latency SEPs described in the preceding paper, the long-latency potentials were more variable and were more affected by intraoperative conditions.
