Abstract

Extinction is a fundamental learning and memory process that enables humans and animals to survive in the face of shifting environmental conditions. The context-specific nature of extinction learning is demonstrated by the renewal phenomenon, in which responding returns
following a change in context after extinction. Pavlovian fear conditioning procedures have primarily been used to investigate the psychological and neural processes mediating extinction. However, compared to passive defensive strategies, the mechanisms governing the extinction of
active defensive strategies are not well understood. This thesis examined the psychological processes mediating the extinction of both active and passive defensive responses using the shock-probe defensive burying task. We found robust ABA renewal and less marked ABC and
AAB renewal of passive coping behaviours. Active coping strategies linked to conditioned defensive burying did not display renewal, indicating that passive coping strategies are more prone to renewal than active coping strategies.These findings have important implications for
understanding how context influences the extinction of different defensive responses to aversive stimuli. Moreover, this thesis employed an appetitive Pavlovian conditioning procedure to investigate the neural mechanisms mediating the extinction of responding to a discrete sucrose
cue. Using Fos immunohistochemistry and correlational network analysis, we identified the neural correlates and networks associated with the recall vs extinction of responding to a sucrose-predictive Pavlovian cue. Our findings are consistent with those obtained using Pavlovian fear and operant reward-seeking procedures, which have demonstrated a functional dichotomy between the prelimbic (PL) and the infralimbic (IL) cortices of the medial prefrontal cortex. Namely, our results were consistent with the idea that the PL promotes the expression,
while the IL mediates the extinction of conditioned responding. Additionally, we found that the paraventricular nucleus of the thalamus (PVT) plays a role in the recall of appetitive Pavlovian responding, and a neural network including the IL and PVT is active during extinction but not
recall, suggesting that IL projections to the PVT may be involved in appetitive Pavlovian extinction. In support of this hypothesis, additional experiments found that optical stimulation of the IL-to-PVT pathway completely blocked appetitive Pavlovian renewal, while stimulation of
the PL-to-PVT pathway had only modest effects on renewal. In the same experiments, stimulation of the IL-to-PVT, but not the PL-to-PVT, pathway supported self-stimulation, suggesting that this pathway has a reinforcing property. Together, these findings provide novel insights into the neural mechanisms underlying the extinction of responding to appetitive
Pavlovian cues, and they point to the PVT as a critical node in the neural circuitry underlying the extinction of appetitive Pavlovian conditioned responding.