PRESERVATION OF DELAYED RESPONSE FOLLOWING COMBINED LESIONS OF PREFRONTAL AND POSTERIOR ASSOCIATION CORTEX IN INFANT MONKEYS

The preservation of delayed response capacity in monkeys sustaining bilateral ablations of prefrontal cortex in infancy has been attributed to the intactness of the caudate nucleus, since combined prefrontal-caudate lesions in infant monkeys produce severe delayed response deficits at later testing. However, before concluding that the integrity of the caudate is essential, it must be shown that another type of combined lesion sustained in infancy is without substantial effect on later test performance. Since bilateral ablations of parieto-temporo-preoccipital (posterior association) cortex in adult monkeys are known to produce delayed-response deficits, a combined prefrontal-posterior association cortex lesion was judged to be the appropriate control. Accordingly, three infant monkeys were subjected to two-stage bilateral ablations of prefrontal and posterior association cortex prior to postnatal day 18. Testing on a series of delayed response problems was initiated between the eighth and tenth postnatal months. Test performances were found to be essentially equivalent to those of animals sustaining only a prefrontal ablation in infancy. In one preparation, a transient delayed response deficit during initial testing was attributed to minor caudate involvement. Although subtotal ablations in posterior association cortex were found in all cases, the massive lesions that were sustained make it unlikely that this area is functionally involved in preserving delayed response capacity in the early lesioned prefrontal monkey. These results tend to confirm our previous conclusion as to the functional importance of the caudate nucleus, possibly via its capacity to regulate locomotion and attention. © 1969.