The Nonhuman Primate Amygdala Is Necessary for the Acquisition but not the Retention of Fear-Potentiated Startle

Background: In a previous study, we found that rhesus monkeys prepared with bilateral lesions of the amygdala failed to acquire fear-potentiated startle to a visual cue. However, a second group of monkeys, which received the lesion after training, successfully demonstrated fear-potentiated startle learned prior to the lesion. Methods: In the current experiment, the eight monkeys used in the second part of the original study, four of which had bilateral amygdala lesions and the four control animals, were trained using an auditory cue and tested in the fear-potentiated startle paradigm. This test was performed to determine whether they could acquire fear-potentiated startle to a new cue. Results: Monkeys with essentially complete damage to the amygdala (based on histological analysis) that had retained and expressed fear-potentiated startle to a visual cue learned before the lesion failed to acquire fear-potentiated startle to an auditory cue when training occurred after the lesion. Conclusions: The results suggest that while the nonhuman primate amygdala is essential for the initial acquisition of fear conditioning, it does not appear to be necessary for the memory and expression of conditioned fear. These findings are discussed in relation to a network of connections between the amygdala and the orbitofrontal cortex that may subserve different component processes of fear conditioning.