Zotow, Ewa; (2019) Neural representational similarity in episodic and spatial memory. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Electrophysiological recordings in rodents and humans show that the contents of spatial and episodic memories are encoded in patterns of activity across neural populations. Here, I applied a mixture of univariate and multivariate techniques to functional magnetic resonance imaging (fMRI) data to investigate memory processes as well as look at how activity across voxels relates to the way memories are represented in the brain. In the first part of the thesis, I applied the Representational Similarity Analysis (RSA) to investigate the brain representations underlying spatial memories. I examined fMRI data from participants during actual or imagined navigation in virtual environments. Evidence for grid cells, one of the principal types of spatial cells, has been reported in single-cell recordings and in univariate approaches to fMRI data. I found evidence for viewing direction within occipital areas in both modalities, suggesting involvement of the same system in both tasks, but not for the six-fold symmetry characteristic of the grid cell signal. I discuss the potential reasons for this null result. In the second part I looked at episodic memories, in particular whether similar information undergoes ‘pattern separation’ during encoding to minimise future interference. I designed tasks using face morphing stimuli and multi-element events with overlapping elements to look for behavioural evidence for pattern separation and interference. The overlapping events showed increased independence in performance across multiple retrievals, suggesting the formation of more independent representations compared to unrelated events. I then looked for neural signatures of this effect using fMRI RSA. I found evidence for a decreased similarity of patterns of overlapping events at retrieval but not at encoding, possibly due to larger influence of perceptual similarity at encoding. Increased pattern separation was not related to decreased interference (improved performance). Lastly, I found evidence of non-target reinstatement, consistently with holistic representation of memory episodes. Overall, this thesis presents new findings as well as replications of previously observed effects using a range of novel behavioural and analysis techniques to investigate how spatial and episodic memories are represented in the human brain.

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