Effects of hippocampal lesions on acquisition and memory for context
Beange, Iona Margaret
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Hippocampal lesions impair memory for context in some tasks but not others. Factors that may contribute include: a) whether context is encoded in configurally or elementally; b) whether lesions are performed before or after acquisition of contextual information. c) the size of the lesion. This study compared the effects of pre- vs post-acquisition hippocampal lesions on performance of a novel context-dependent odour discrimination task that required explicit processing of the contextual features. As the task required a configuration to be formed between context, odour and reward, it was hypothesised that the hippocampus would be essential for the acquisition and performance of this task. Pre-surgery training consisted of simultaneous presentations of a context-dependent and a context-independent odour discrimination task. In the context dependent task, odour A but not odour B was rewarded in context 1, whereas odour B but not odour A was rewarded in context 2. In the context independent task, odour C was rewarded in both contexts, whereas odour D was rewarded in neither. Rats took around 60 days to reach criterion level (2 days >80% correct on both tasks). Subsequently, they received either bilateral ibotenic acid lesions of the hippocampus or sham surgery. After a 14 day recovery period, post-surgery testing began. On the first 2 days of post-operative testing, lesioned animals were significantly impaired on the CD task, but not on the CI task. Thereafter they performed as well as controls. Thus, the data demonstrate that although the hippocampus normally contributes to the retention of contextual information, it is not necessary for the performance of this context dependent odour discrimination task. Other areas can take over these functional demands in its absence. However, the involvement of the hippocampus cannot be completely disregarded due to the high degree of correlation between spared hippocampal tissue and the immediate post-surgery performance level of the animals (i.e. larger the volume of tissue spared the higher the initial degree of accuracy on the CD task). These findings were shown to be highly replicable, regardless of whether the odorous stimuli were presented simultaneously or successively. Furthermore, the hippocampal and extra-hippocampal methods of task resolution were not identical. When a cue conflict situation arose between intra-maze and selfmotion cues, it affected the two groups in a differentially. The ambiguity between cues had a highly detrimental affect on the performance of the intact animals; yet the hippocampal lesioned animals appeared oblivious to the inconsistency. They continued to perform the context dependent odour discrimination task as normal. Thus although apparently able to process the major contextual cues, the hippocampal lesioned animals had a deficit in detecting and responding to more subtle distinctions that were not integral to normal success on the task. In the final aspect of this thesis, hippocampal lesioned animals were found to demonstrate no deficits in the acquisition of new variants of the context dependent odour discrimination task (new odours / contexts), thus the hippocampus is not essential for learning contextual discriminations. Overall, the hypothesis that the hippocampus would be necessary for contextual representations, is unsupported by this thesis. Nevertheless, if present during training, the hippocampus will contribute to the retention of contextual stimuli and provides a more all encompassing view of ‘context’ than other areas can achieve alone.