Effects of Frontal Cortex Lesions on Object Discrimination Learning by Monkeys

Abstract

FOR a long time it has been known that monkeys with bilateral dorso-lateral frontal cortex lesions are impaired on delayed response problems. At the same time there does not generally appear to be any impairment on visual discrimination learning. Recently, Rosvold and Mishkin¹ have suggested that, in certain circumstances, the discrimination learning of monkeys with frontal lesions may be influenced by various preference factors. Specifically, Brush et al.² reported that monkeys with frontal lesions are impaired, as compared with intact animals and others with inferotemporal lesions, on a series of object discrimination problems. Each individual problem was presented for ten trials. Bait was placed under both objects on the first trial of half the problems, and the object chosen by the monkey became the positive (rewarded) cue on the subsequent trials. On the first trial of the remaining problems bait was not placed under either object; the chosen object became the negative (unrewarded) cue on the subsequent trials. The frontal monkeys were impaired particularly on this latter category of problem—that is, where a reward could be obtained only by responses to the initially non-preferred object. It was this finding which led Rosvold and Mishkin to their conclusion that preference factors were an important feature of the frontal deficit.