Author Type

Graduate Student

Date of Award

Spring 4-30-2026

Document Type

Dissertation

Publication Status

Version of Record

Submission Date

May 2026

Department

Biological Sciences

College Granting Degree

Charles E. Schmidt College of Science

Department Granting Degree

Biological Sciences

Degree Name

Doctor of Philosophy (PhD)

Thesis/Dissertation Advisor [Chair]

Yingxue Wang

Thesis/Dissertation Co-Chair

Robert W. Stackman Jr.

Abstract

A critical component of spatial navigation is the ability to estimate elapsed time or distance traveled. How the brain generates such estimates and links them to behavior remains unclear. In this thesis, we identify a previously unrecognized population coding strategy for time or distance in hippocampal CA1, a region critical for navigation and memory. Using novel behavioral paradigms in head-fixed mice combined with electrophysiological recordings, we show that this code is carried by two functionally distinct subpopulations of pyramidal neurons, termed PyrUp and PyrDown, whose firing rates evolve in opposite directions during navigation. Both populations exhibit a common two-phase dynamic: a rapid, synchronized change in activity at navigation onset, followed by gradual ramps or decays that unfold at heterogeneous rates across neurons. The slower second phase of PyrUp/PyrDown dynamics rescales proportionally across environments of different sizes, supporting flexible representations of elapsed time or traveled distance. Optogenetic suppression of CA1 activity further reveals that this second phase, but not the initial rapid phase, is necessary for task performance. We show that the point at which declining PyrUp activity intersects with rising PyrDown activity consistently precedes the onset of reward-seeking behavior, suggesting that relative activity between these subpopulations helps determine when actions are initiated during navigation. Finally, we identify local circuit mechanisms that support these dynamics. Optogenetic inactivation of somatostatin-positive(SST+) interneurons reduced the ramping activity of PyrUp neurons, leading mice to prematurely initiate reward collection, suggesting impaired time or distance estimation. Conversely, inactivation of parvalbumin-positive(PV+) interneurons prevented PyrDown neurons from shutting off at the onset of navigation, leading to transient attempts at reward collection around this point. These findings reveal parallel hippocampal circuits that initiate and maintain time or distance encoding, controlled by PV+ and SST+ interneurons, respectively. Collectively, this work defines a CA1 population code for time and distance and the circuit mechanisms through which this code shapes decision-making during navigation.

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