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Abstract
Research into the neuroanatomical bases of long-term memory has consistently implicated medial temporal lobe structures, namely the hippocampus, to play a critical role in the consolidation of newly acquired episodic memory (memory of specific events in a person s life) for long-term storage. The work described in this thesis investigates consolidation of anterograde episodic memory using recognition and source (contextual) behavioural tasks with the addition of extensive neuroimaging techniques to better understand the role of medial temporal lobe structures when this process is intact and when it is impaired. We examined intact consolidation and the cognitive effect of healthy ageing in a sample of young and elderly participants, and impaired consolidation in early stage Alzheimer s disease (AD) and semantic dementia (SD) patients, who show contrasting atrophy in the medial temporal lobe region.
Building on previous studies, functional neuroimaging revealed the posterior region of the left hippocampus is crucial for intact long-term consolidation of memories that have a contextual component. Diffusion tensor imaging further revealed greater white matter integrity in the fornix and cingulate, two subcortical structures responsible for efferent and afferent communication with the hippocampus, respectively, also predicted successful consolidation. Behaviourally, cognitive changes associated with healthy ageing showed decreased recognition and source memory retrieval after extended delays, but the same pattern of performance change over time was observed in young and elderly participants. In amnesic patients, AD showed a significant dissociation in the pattern of source memory performance over time compared to controls while in SD this was only observed for recognition memory. Considering atrophy in early stage AD is often reported to be confined to the hippocampus, these results are consistent with the multiple trace model of long-term consolidation.
The critical role of the hippocampus and adjacent subcortical structures along the Papez memory circuit in long-term consolidation of anterograde episodic memory is consistent with our findings. In addition, our results from additional testing of a single SD patient found the memory impairment appears to leave implicit long-term memory retrieval intact. The theoretical and clinical implications of these findings for human long-term memory will be discussed.