Williams, Merlin; (2023) Single Neuron Encoding of Head Orientation in the Rat Brain. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

Head direction cells (HDCs) are thought to represent the azimuthal direction that an animal’s nose is pointing in a one-dimensional, compass-like, fashion. The underlying mechanisms of head direction cells in freely moving animals traversing continuously varying three-dimensional (3D) surfaces has yet to be explored. In this work, I recorded HDCs electrophysiologically while rats foraged atop a hemispherical arena. 3D head orientation data was recorded along with 3D positional data. I found that previous models were unable to account for variability in preferred firing direction (PFD) of recorded HDCs on the hemisphere, including when testing whether the HD representation was tied to the locomotor plane, or the angle of the head. A number of HDCs formed swirl patterns on the hemisphere; their swirl directions were highly lateralised. I present two novel frameworks in which HDCs are recontextualised as components of a system for representing the 3D orientation of the head. I apply these frameworks to my data, as well as previously published results, showing that one interpretation better explains the firing patterns of HDCs. Further, I describe generative models of my frameworks and use them to simulate individual cells – some of which would appear as traditional HDCs if analysed conventionally. I conclude that cells in the HDC system could represent the 3D orientation of the head, and suggest that this same mechanism could represent the orientation of other, external, objects as components of an internal model of the world.

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