What information is represented in the human hippocampus?

• Main Author: Chadwick, M. J.
• Published: University College London (University of London) 2012
• Subjects: 612.8
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.565748

The hippocampus plays a critical role in supporting memories of our personal past experiences (episodic memories). However, it is not known how individual episodic memories are represented by neuronal populations within the hippocampus. The aim of my thesis was to explore the nature of the information represented in the human hippocampus, with a particular focus on episodic memories. I conducted five experiments using high-resolution and standard functional MRI (fMRI). In four of these projects I used and further developed a method known as multi-voxel pattern analysis (MVPA). This enabled me to interrogate the fMRI data to look for functionally-relevant patterns of information encoded across multiple voxels. My findings revealed that episodic memories were represented in the hippocampus more so than in neighbouring brain regions, that this was true even of memories that were highly overlapping in terms of content and context, and for recently-formed and very remote memories. Furthermore, I found that the episodic information within individual hippocampal subfields was consistent with computational models of episodic memory. One important contribution to the representation of an episodic memory is scene construction - the mental construction of a complex spatially coherent scene into which event details are bound. In order to explore the role of the hippocampus in scene construction, I used fMRI to study boundary extension – a scene-related phenomenon whereby people extrapolate beyond the edges of a given view. This revealed that hippocampal activity tracked the emergence of boundary extension, suggesting that scene construction can be rapid, automatic, and implicit. Overall, my findings shed new light on the nature of episodic representations within the human hippocampus, and offer an empirical link between episodic memory and computational theory. Moreover, they provide further evidence regarding scene construction, which is a key component of episodic representations within the hippocampus.