Date of Award
2025
Document Type
Thesis (Ph.D.)
Department or Program
Psychological & Brain Sciences
First Advisor
Matthijs van der Meer
Abstract
This thesis investigates the viability and generalizability of two prominent oscillation-based theories in the rodent limbic system: “communication through coherence” (CTC) in the ventral striatum (vStr) and “theta sequence” coding in the hippocampus. First, we tested fundamental physiological requirements for the CTC hypothesis, which posits that the vStr flexibly routes convergent inputs by modulating oscillatory coherence. We used simultaneous optogenetics and electrophysiology in awake, head-fixed mice to test the first requirement: phase-dependent excitability. We found that a third of vStr PV+ interneurons exhibit the hypothesized phase-dependent excitability, providing the first direct physiological evidence for this foundational component of CTC in the local circuitry. Next, we tested a second requirement for CTC : the existence of a "flexible switching mechanism." We analyzed spiking and LFP data from rats, testing the in vitro-based hypothesis that Medium Spiny Neurons (MSNs) act as a switch by systematically changing their phase-locking frequency with their firing rate. Our results did not support this specific hypothesis. MSNs showed highly heterogeneous, non-systematic changes, refuting this proposed mechanism. We then moved to the hippocampus to test the generalizability of "theta sequence" coding to the non-spatial domain. We recorded dorsal CA1 neurons in head-fixed mice that were passively exposed to predictable and random odor sequences. Despite robust encoding of the current odor identity in both blocks, we found no evidence of the predicted "Past-Current-Next" theta sequence in the predictable block. Subsequent analyses using novelty and sequence-violation probes confirmed a lack of neural evidence for sequence learning. This thesis provides a mixed verdict on vStr CTC, confirming a key component while refuting a specific switching mechanism. Collectively, our findings suggest that these canonical oscillation-based theories are not a default processing mode, but are likely conditional mechanisms engaged only when the temporal structure of information becomes behaviorally relevant.
Recommended Citation
Mohapatra, Manish, "Viability and Generalizability of Canonical Oscillation-based Theories in the Rodent Ventral Striatum and Hippocampus" (2025). Dartmouth College Ph.D Dissertations. 436.
https://digitalcommons.dartmouth.edu/dissertations/436
