Relative contributions of SII and area 5 to tactile discrimination in monkeys

Rhesus monkeys with ablations of either the second somatosensory cortex (SII) or of the superior parietal lobule (area 5) were tested on a battery of tactile discrimination tasks in order to help determine which of these areas might constitute part of a postulated cortico-limbic tactile processing pathway. Monkeys with ablations of SII were severely impaired on both texture and shape discrimination learning and had markedly elevated size and roughness discrimination thresholds relative to control animals. By contrast, monkeys with area 5 lesions were impaired only on roughness thresholds, and these were elevated only moderately. Although more severe tactile deficits following lesions of area 5 have been reported previously, they were found in the present study only when the area 5 removals were extended slightly rostrally, in a third operated group, to include the posteromedial part of the hand representation of area 2. These results are consistent with the suggestion that SII, but not area 5, is a critical station in a tactile processing pathway that proceeds from the primary somatosensory cortex (SI) to the limbic structures of the temporal lobe through links in SII and the insular cortex.