The neural correlates of Human Spatial Memory and Representation
Murphy, Jonathan Scott (2009) The neural correlates of Human Spatial Memory and Representation. PhD thesis, National University of Ireland Maynooth.
The importance of object-location memory for our everyday survival is now well accepted. This behaviour relies on spatial representation and memory within the brain. With these representations, we are able to construct what has been described as a “cognitive map” (Tolman, 1948) that allows us to accurately direct ourselves within our environment, be it throughout a city or within a building, in an automobile or on foot. However, how these representations interact remains poorly understood. In particular, the temporal dynamics involved in the recruitment of and retrieval from different spatial representations has received little attention. Likewise, the use of spatial information for object-location binding is another largely unexplored area. As virtual reality begins to gain a solid foothold in psychology laboratories, we describe a novel and flexible small-scale test of spatial memory which we coined ‘The Spatial Grid Task’. With variants of this array task we investigated human spatial memory in an attempt to test the relationship between elements of the cognitive map and between spatial and object memory. This investigation has involved a number of experiments studying the efficacy of different information-types on performance and analysis of performance from shifted-viewpoints as well as an examination of the neural correlates of spatial memory. We report behavioural and electrophysiological differences in ego- and allocentric strategies and provide the first temporal markers identifying the divergence between representations. Amplitude differences in a parietal P300 component are found to emerge after 300ms with evidence that early translational processes precede location categorisation processes. These differences were found to be consistent after controlling for task difficulty, mental rotation, scene recognition and other ecological confounds. In addition to interactions within the cognitive map, we assessed how, when and where in the brain spatial information is integrated with object information. Our investigations used implicit and explicit measures, both of which revealed a locational bias in information-processing. Electrophysiological differences suggest that spatial evaluation can exhibit an early (and implicit) influence on object recognition. Explicitly, the primacy of spatial processing was accompanied by earlier peaking frontal P2 components and centro-parietal P300s when participants were evaluating locations compared to objects. The results are discussed alongside models of brain activation; these models suggest structures that are dissociable along the ventral and dorsal streams as well as highlighting areas of convergence. The parahippocampal gyrus of the MTL is posited to play a crucial role in spatial coding while more dorsal regions and the posterior cingulate cortex are suggested to underlie integration and translation. This thesis details experiments which are amongst the first to use EEG to probe spatial memory in such detail as to expose electrophysiological differences between representations. As well as showing viewpoint-related differences, the work suggests areas that are engaged for translation between representations and provides temporal markers for their involvement. It also gives an insight into the processing speeds along the visual streams suggesting a contextual dominance in object-location (and episodic) memory. Finally, this thesis provides clear electrophysiological markers of spatial memory which can be used in further research with normals and in the assessment of brain damage.
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