Hippocampal-dependent spatial and episodic memory in humans

The role of the human hippocampus in spatial and episodic memory was investigated using methods from neuropsychology, experimental psychology and functional magnetic resonance imaging (fMRI). The provision of allocentric representations in spatial memory and context-dependent memory for episodic events were each investigated, both for themselves and with the aim of identifying any commonalities in hippocampal function. Inspired by the similarity of human memory for location and the allocentric representations in the hippocampi of freely moving rats, a difficulty-matched spatial memory test was developed to compare allocentric performance requiring viewpoint- independent representations of location with performance for which viewpoint-dependent representations suffice. It was shown that hippocampal damage leads to a specific impairment in the allocentric condition. This test also indicated that topographical disorientation can result from a selective deficit in allocentric spatial memory, and is under continued development for early detection of Alzheimer's disease. Episodic memory was investigated in three ways. First, the way in which different aspects of an event are bound together, holistic or fragmented, was investigated using a virtual reality test of context-dependent memory. We found fragmented representations and distinct retrieval hierarchies. Second, the hippocampal involvement in personally relevant memories was investigated with fMRI, using personally known, famous and unknown faces as cues. Extensive hippocampal activation was found for both personally known and famous faces, most likely reflecting the vividness and semantic embedding of memories cued by these stimuli. Third, hippocampal involvement in multimodal context-dependent memory was investigated using fMRI of memory for events occurring in different spatial and olfactory contexts. Extensive, partially overlapping, hippocampal activation was associated with retrieval of both types of context. The results are discussed in terms of the hippocampal role in memory, its laterality, and the relationship between viewpoint independence, multi-modal context and semantic embedding in retrieval