Murphy, Jonathan Scott
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
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