DIFFERENTIAL-EFFECTS OF AMYGDALA LESIONS ON EARLY AND LATE PLASTIC COMPONENTS OF AUDITORY-CORTEX SPIKE TRAINS DURING FEAR CONDITIONING

In auditory fear conditioning, pairing of a neutral acoustic condition ed stimulus (CS) with an aversive unconditioned stimulus (US) results in an enhancement of neural responses to the CS in the amygdala and au ditory cortex. It is not clear, however, whether cortical plasticity g overns neural changes in the amygdala or vice versa, or whether learni ng in these two structures is determined by independent processes. We examined this issue by recording single-cell activity in the auditory cortex (areas Te1, Te1v, and Te3) of freely behaving, amygdalectomized rats using a movable bundle of microwires. Amygdala damage did not af fect short-latency (0-50 msec) tone responses, nor did it interfere wi th conditioning-induced increases of these onset responses. In contras t, lesions of the amygdala interfered with the development of late (50 0-1500 msec) conditioned tone responses that were not present before c onditioning. Furthermore, whereas onset conditioned responses in the c ontrol group remained elevated after 30 extinction trials (presentatio n of CS alone), onset responses in lesioned animals returned to their preconditioning firing level after approximately 10 extinction trials. These results suggest that the amygdala enables the development of lo ng-latency (US anticipatory) responses and prevents the extinction of short-latency onset responses to threatening stimuli. The findings fur ther suggest that auditory cortex cells may participate differently in explicit and implicit memory networks.