Title: Electrophysiology of Human Spatial Navigation and Memory
Description: The question of how we form memories has fascinated scientists for decades. The hippocampus and surrounding medial-temporal-lobe (MTL) structures are critical for both memory and spatial navigation, yet we do not fully understand the neuronal representations used to support these behaviors. Much research has examined how the MTL neurally represents spatial information, such as with “place cells” that represent an animal’s current location or “head-direction cells” that code for an animal’s current heading. In addition to attending to current spatial locations, navigating to remote destinations is a common part of daily life. In this dissertation I investigate how the human MTL represents the relevant information in a goal-directed spatial-memory task. Specifically, I analyze single-neuron and local field potential (LFP) data from neurosurgical patients with respect to their spatial navigation and memory behavior, with a focus on probing the link between neuronal firing, oscillations, and memory.

In Chapter 2, I find that the firing rates of many MTL neurons during navigation significantly change depending on the position of the current spatial target. In addition, I observe neurons whose firing rates during navigation are tuned to specific heading directions in the environment, and others whose activity changes depending on the timing within the trial. By showing that neurons in our task represent remote locations rather than the subject’s own position, my results suggest that the human MTL can represent remote spatial information according to task demands. In Chapter 3, I find that during encoding the left hippocampus exhibits greater low theta power for subsequently recalled items compared to unrecalled items. I also find that high frequency activity and neuronal firing in the hippocampus distinguish between item-filled compared to empty chests. Finally, I find that MTL cells’ firing rates and the differential timing of spikes relative to low frequency oscillations in the LFP distinguish between subsequent recall conditions.

These results provide evidence for a distinct processing state during the encoding of successful spatial memory in the human MTL. Overall, in this thesis I show new aspects of the neural code for spatial memories, and how the human MTL supports these representations.