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MANY STUDIES have shown that bilateral ablation of the frontal granular cortex in monkeys is followed by inability to perform at better than a chance level on the delayed-response test, even at delays of less than five seconds. Evidence for more precise localization of lesions within the prefrontal region which produce this symptom, however, is scant, elusive, and contradictory. In 1935 Jacobsen1found that after complete bilateral removal of all prefrontal cortex in the monkey except for a strip "several millimeters in width" anterior to the premotor area on one side only, the resulting deficit was quantitatively incomplete and was expressed as a shortening of the length of delay after which the animal could respond accurately. From this experiment, he suggested that the degree of deficit is inversely related to the amount of intact tissue of the frontal lobes. A similar experiment on a baboon, in which there was
Robert A. Blum (Sat,) studied this question.