DISCRIMINATION REVERSAL AFTER LATERAL FRONTAL LESIONS IN MONKEYS1

Monkeys impaired on delayed response following midlateral frontal lesions, control operatees, and unoperated Ss were tested on 2 discrimination re-versals. In Experiment 1 Ss were repeatedly tested for reversal of a dis-crimination within a session and retention of the same discrimination over 24 hr. Each reversal phase and each retention phase consisted of 30 trials. The frontal operatees made more errors than the other Ss in the reversal phases but not in the retention phases. In Experiment 2 Ss were trained to reverse a previously well-established visual habit. The groups overlapped on this task; each required a mean of over 100 trials to reach criterion. The re-sults suggest that midlateral frontal lesions produce a discrimination deficit that is inversely related to the number of trials per problem. In the macaque, frontal lesions which in-clude sulcus principalis severely impair per-formance on delayed response tasks. This type of lesion also produces impairment o