Abstract
The process of forming and retrieving mental links between cues, actions, andoutcomes is known as associative memory. This cognitive function critically impacts
how animals use past experiences to guide their decision making, shaping their
interactions with their environment. However, it is unclear what brain mechanisms
underlie the retrieval of these associative memories when they are needed, and how
these memories are translated into behavior. It is widely accepted that associative
memory and decision making involve brain-wide networks comprising sensory and
cognitive regions, with key roles for the hippocampus and prefrontal cortex (PFC).
Coordination in the hippocampal-prefrontal network is known to be crucial for a variety
of mnemonic processes, but the specific coordination mechanisms that enable recallguided
decision making are not understood.
In this thesis, I describe experiments to address this question in which we
monitored activity in olfactory, hippocampal, and prefrontal regions in rats performing an
odor-cued spatial associative memory task. In this task, rats had to recall familiar odorvii
place associations and use these memories to execute correct spatial decisions to
obtain rewards. Decision making occurred immediately following odor-cue sampling,
indicating that the odor sampling period represents a temporal window for recall- and
decision-related processing. During this period, the respiratory rhythm (RR, 7-8 Hz) and
the beta rhythm (15-30 Hz) were prominent and coherent across the three recorded
brain regions. Coherence in the beta band, but not RR, was stronger on correct trials,
suggesting beta oscillations coordinate the sensory and cognitive elements of the
system for accurate recall-guided decision making. Furthermore, single neurons and
ensembles in the hippocampal-prefrontal network exhibited choice-coding activity and
predicted upcoming choice. These dynamics were linked specifically with beta rhythm
coordination but not with RR. After a decision was made on each trial, choice selectivity
was maintained independently by separate theta-modulated ensembles during running
on the rest of the maze, suggesting a specific role for beta-modulated ensembles in
recall-guided decision making. Despite a significant relationship between beta
coherence and the emergence of task-related ensemble dynamics, there was little direct
relationship between beta phase-locking and choice-selective coding in putative
pyramidal populations. Rather, interneurons in hippocampus were strongly modulated
by local and cross-regional beta, suggesting that beta-governed interneuron activity
may shape task-related ensemble activity underlying memory-guided decision making.
Together, the results presented here suggest that beta-mediated network and
ensemble coordination in the hippocampal-prefrontal network underlies cued recall and
utilization of odor-place associations. These findings provide insight into how associative memories are translated into behavior and offer additional avenues for
research to further elucidate this process.