Brain Regions Involved in Long-term Spatial Memory: fMRI and Behavioural Studies

In this thesis, I investigated the role of the hippocampus and other brain regions in long-term spatial memory. I used neuroimaging and behavioural techniques to compare spatial representations that are dependent on the hippocampus and those that rely on extra-hippocampal structures. In Experiment 1, I demonstrated that at least some spatial memories that are initially dependent on the hippocampus can become independent of it with time. This was done by looking at changes in brain activation as participants became familiar with a real-world environment over a year. Hippocampal activation was found during mental navigation tasks when participants were new to Toronto, but not after they had lived there for a year. This change was accompanied by an increase in activation in the posterior parahippocampal gyrus, lingual gyrus, cuneus, and superior temporal gyrus. In Experiment 2, I used neuroimaging to compare hippocampal involvement during the retrieval of coarse- and fine-grained spatial details, and episodic details associated with a familiar environment. I showed that hippocampally-mediated representations contain more fine-grained spatial details than extra-hippocampal spatial representations. Further, I demonstrated that fine-grained details become less dependent on the hippocampus with experience, but episodic details require the hippocampus throughout the lifetime of the memory. In Experiment 3, I provided behavioural evidence that the role of hippocampus in episodic memory associated with a familiar environment is crucially related to the degree of detail retrieved. Older adults were asked to recall walking routes from their daily lives and the number of details retrieved was correlated with tests sensitive to hippocampal function. Finally, I showed that the extra-hippocampal regions implicated in spatial memory depend on task demands. Basic navigation can be supported by the posterior parahippocampal gyrus, lingual gyrus, superior temporal gyrus, caudate and inferior frontal gyrus, independently of the hippocampus. When navigation requires fine-grained spatial details, additional regions including the precuneus and supramarginal gyrus will be recruited. When the task requires the retrieval of episodic details, the posterior cingulate, angular gyrus, and medial frontal lobes will be required, along with the hippocampus.Ph