Frontal decortication in rhesus monkeys: a test of the interference hypothesis.

Four monkeys with dorsolateral frontal ablations and 3 unoperated controls were run on discrimination problems which sequentially presented both relevant and irrelevant visual stimuli prior to the opportunity for a choice response. As previously reported, monkeys with undamaged brains performed significantly better on those problems presenting relevant information first, being unaffected by later occurring irrelevant stimuli. Contrary to the behavior of the normal monkeys, monkeys with lesions of the dorsolateral frontal cortex were severely impaired when irrelevant stimuli were presented after the relevant stimuli had been processed, but before the opportunity to respond had occurred. In other words, the frontal monkeys performed just the opposite of the normals in these procedures, even though the relevant and irrelevant stimuli were manipulated within the usual temporal definitions of the two-choice discrimination trial. These data demonstrate that the presentation of irrelevant stimuli before the choice response can significantly impair frontally decorticated monkeys and that this impairment does not require the use of a long temporal delay preceding the opportunity to respond. For this reason the data were interpreted as rather direct support of the interference hypothesis of frontal dysfunction. On the basis of these and other data discussed, it was concluded that one function of the dorsolated frontal cortex involves the suppression of interfering stimulus events interposed between the information necessary to guide behavior and the behavior itself.