Abstract:
Much of what is known about the brain mechanisms of fear comes from tracing neural pathways of individual component behaviors. The particular component behavior examined in this thesis is conditioned defensive freezing in the rat ('Rattus norvegicus'). Experiment 1 (lesions before conditioning) looked for differences in extinction after dlPAG lesions in rats that received context conditioning with either massed or distributed shock Experiment 2 (lesions after conditioning) determined the effects of dlPAG after the rats had already experienced the context-shock pairings. Finally, Experiment 3 examined freezing and analgesia in dlPAG lesioned and sham rats that had received either 1 or 3 (massed) shocks. The results of these experiments support Fanselow et al.'s (1995) increased learning hypothesis. Specifically, the results showed that (a) dlPAG lesions placed before, but not after, conditioning facilitated fear conditioning, (b) dlPAG lesions did not influence the level of conditioned fear supported by a single shock, and (c) these effects do not appear to be mediated by differences in pain sensitivity or in the magnitude of the unconditioned response to the shock reinforcer. (Abstract shortened by UMI.)
