UC San Diego

ABSTRACT OF THE THESIS

Investigating the Relationship Between Hippocampal Theta Oscillations and Firing Properties of CA1 Time Cells

byAmeen Khan Master of Science in Biology

University of California San Diego, 2021

Professor Stefan Leutgeb, Chair

Hippocampal place and time cells are thought to play a major role in the encoding of the spatial and temporal components of episodic memory. Extensive research in rodents has been performed studying the firing properties of hippocampal place cells. However, the firing properties of more recently discovered time cells are still not well understood. In this study, we performed electrophysiological recordings from CA1 time cells in rats as they performed a spatial working memory task on a figure-8 maze. The goal of this study was to better understand how differences in both the duration of the delay and running behavior influence the sequential activity of time cells. Time cells were recorded across two different delays of either 10 s or 30 s. The figure-8 maze was modified to contain an electric treadmill in the delay zone. The electric treadmill served to control behavior of rats during the delay zone and, depending on the extent of running, resulted in reduced theta oscillations. We found that a smaller proportion of time cells coded for later portions of the delay period. These late time cells were less stable both within and across corresponding blocks. However, when considering the entire population of time cells, CA1 time cell activity seemed to be stable between corresponding blocks irrespective of the delay length and the extent of theta oscillation during the delay period. Furthermore, the proportion of time cells did not differ depending between forced running blocks and blocks with the treadmill off, which suggests that the continuity of theta oscillations during the delay period was not necessary for the maintenance of time cell activity. Our experiment provides further insight into the mechanisms that govern the sequential activity of time cells.