Contributions of the hippocampus, perirhinal cortex, and amygdala to object-fear conditioning

Fear conditioning refers to the ability to learn and remember that a stimulus predicts the occurrence of a fear-eliciting event. The neurobiology of fear conditioning to auditory, olfactory, and visual stimuli and to contexts has been extensively studied, but the neurobiology of fear conditioning to an object has not been thoroughly investigated. Thus, the focus of the present thesis was to determine whether object-fear conditioning depends on the hippocampus, perirhinal cortex, and amygdala. It was hypothesized that pre- and post-training lesions to either structure in rats would cause amnesia for an association between an object and a fear-eliciting event because each is involved in object recognition and/or fear conditioning. Accordingly, rats' memory was tested in the shock-probe fear-conditioning test, which required them to remember that a small plastic probe protruding from the bottom of a wall of a testing chamber was associated with a mild shock. Several behaviours, such as burying, avoidance, and changes in stress hormone were assessed during the test to comprehensively evaluate fear conditioning. It was found that hippocampal lesions caused both anterograde and retrograde amnesia. However, the anterograde amnesia resulted from a contextual fear-conditioning deficit, whereas the retrograde amnesia resulted, at least in part, from an object-fear conditioning impairment. By contrast, it was found that perirhinal cortex lesions did not cause memory deficits. Albeit, pre and post-training perirhinal cortex lesions altered avoidance and burying evoked by the object and context during the test, suggesting that the latter structure is likely involved in appraising stimuli that guide fear responses. It was also found that amygdala lesions caused retrograde, but not anterograde, amnesia for object-fear conditioning. The findings are discussed within the context of current theory of the neural basis of cued and contextual fear conditioning and it is concluded that object-fear conditioning involves memory systems that may be distinct from those of other types of fear conditioning