This dissertation examined the role of rat perirhinal cortex (PR) in the neurobiology of emotional learning and memory using classical delay and trace auditory fear conditioning. Damage to PR profoundly impairs delay fear conditioning to 22 kHz ultrasonic vocalizations (USVs), but has little effects on delay fear conditioning to continuous tones. Twenty-two kHz USVs are ethologically-relevant social signals that consist of strings of discrete calls of varying durations. Two different approaches were used to test the hypothesis that temporal discontinuity is the key stimulus feature of USVs in contributing to PR-dependent fear learning. The first study (Chapter 2), using a differential delay fear conditioning paradigm, found that rats are able to discriminate between two auditory stimuli based on temporal discontinuity or principle frequency. The second study (Chapter 3) demonstrated that neurotoxic lesions of PR result in severe impairment in delay fear conditioning to 53 kHz discontinuous cues, but have no effect on fear conditioning to a frequency-matched continuous tone. These findings add strength to the inference that PR normally mediates or enables the fusion of discontinuous auditory stimulus segments into unitary representations or "auditory objects".

Lesions of PR also impair trace fear conditioning, but the underlying mechanism is unclear. The third study (Chapter 4) tested a recent hypothesis that trace conditioning is supported by muscarinic-dependent persistent firing in PR neurons. Blockade of muscarinic receptors within PR selectively disrupted trace fear conditioning but not delay fear conditioning. The results indicate that persistent firing activity within PR might be a viable cellular substrate for aspects of transient memory. Overall, the present studies suggest that PR has multiple functions in associative learning and memory, which may be mediated by different neurotransmitter systems.