Learning in rhesus monkeys after amygdalectomy in infancy or adulthood

This study examined the learning abilities of mature rhesus monkeys that had been subjected to bilateral amygdalectomy either during infancy or adulthood. At 4 1/2 years of age, 6 female monkeys that had been amygdalectomized in infancy and 12 neurologically intact controls were trained on a barpress avoidance problem. At 6 years, half the controls were amygdalectomized. At 7 1/2 years, the infant-operated, adult-operated and control monkeys were trained on shock-motivated problems, including a panelpress avoidance and 15 position reversals, and food-motivated problems, including delayed alternation and 15 position reversals. Infant-amygdalectomized monkeys required significantly more sessions than controls to meet the barpress avoidance criterion at 4 1/2 years, and they also took longer to extinguish the response. At 7 1/2 years, neither the infant- nor the adult-operated monkeys exhibited any difficulty with the shock-motivated problems. On the food-motivated problems, however, both amygdalectomy groups made more preservative errors than controls. The cumulative results from this study and from previous studies with these animals suggest that amygdalectomy in infancy produces no long-term sparing of function in monkeys. When infant- and adult-operated monkeys are tested at the same age, their deficits are indistinguishable.