VARIABILITY OF DELAYED-REACTION PERFORMANCE IN NORMAL AND BRAIN-DAMAGED RHESUS MONKEYS

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Abstract
Among 17 intact Macaca mulatta acquisition of direct, spatial delayed reaction followed many courses, no one function being adequately descriptive of the data. In 4 monkeys with preoperative criterional training on the test, removal of the lateral premotor cortex in both hemispheres had no significant effect. Four monkeys with bilateral prefrontal lesions showed the usual initial postoperative deficit, and the speed of recovery was inversely related to the speed of preoperative learning. The hypothesis that this was due to possible preoperative overtraining of the monkeys that recovered more quickly was examined and rejected. Two more prefrontal monkeys with over 5 years of presurgical behavioral training (including participation in 9 experiments on delayed reaction) also showed impairment. Two test-wise control animals with bilateral temporal lobe ablations had no deficit. For prefrontal monkeys, it was hypothesized that performance may be at least as much influenced by individual differences in mode of solution of delayed reaction as it is by the specific locus and extent of injury within the dorsolateral frontal granular cortex. Other aspects of the experiments indicate that delay interval itself, rather than the period of presentation of the differential cues, is a temporal locus of interfering effects in prefrontal monkeys.