Unilateral removal of the second somatosensory projection cortex in the monkey: evidence for cerebral predominance?

The second somatosensory projection cortex (area SII) was removed unilaterally in 18 monkeys: alone, in 6; with bilateral removals of the supplementary motor area (SMA), in 4; with bilateral removals of anterior parietal association cortex, namely area PF (or 7b), in 4; and with neocortical commissure section, in 4. In half of these monkeys SII was removed from the hemisphere opposite to the preoperatively preferred hand, in half from the hemisphere on the same side as the preferred hand. Histological findings in all 18 monkeys indicated no consistent difference between the lesions (site, extent or depth) of the 9 monkeys with contralateral and the lesions of the 9 monkeys with ipsilateral removals of SII. These 18 monkeys, together with 6 additional unoperated control monkeys, were required to learn to make tactile discriminations, either with their preferred or with their nonpreferred hand. The discriminations were either between pairs of solid objects, or between graded differences in roughness or size. The monkeys with SII removal from the hemisphere contralateral to their preoperatively preferred hand were significantly impaired relative to the ipsilateral group or to the control group irrespective of the hand used (postoperatively preferred or nonpreferred hand). Such impairment was evidenced on the majority (but not all) of the tasks used in this study. In general, removal of SII from the ipsilateral hemisphere did not produce impairment, and bilateral removals of SMA combined with a unilateral removal of SII failed to give rise to additional impairment. However, bilateral removal of area PF, as also commissure section, did produce additional impairment, but only in animals of the ipsilateral group and when the postoperatively nonpreferred hand was required to be used. Certain of the behavior variations within groups can be explained in terms of weak preoperative hand preferences, or by reference to the histological findings for these animals. In general we conclude that the monkey has a degree of cerebral predominance of the hemisphere opposite to the preferred hand, possibly in relation to bilateral somatosensory representation.