Characterized as the ability to remember personal experiences, episodic memory features the learning, storage, and retrieval of spatial, temporal, and object-specific information associated with an event. Dysfunction of episodic memory has long been studied as a source of morbidity in neuropsychiatric disease and a vexation of normal aging. Current theories propose that the neural substrate of episodic memory encompasses medial temporal lobe structures, including the hippocampus, and connections to and between neocortical areas and underlies episodic memory storage and retrieval processes. Of specific interest is the interaction between the medial prefrontal cortex (mPFC) and hippocampus and the roles played by each structure in context-guided memory retrieval. Evidence suggestive of both “top-down” executive control (subserved by the mPFC and its influence over lower cortical and subcortical structures, including the hippocampus) and “bottom-up” control (arising from information flow from the hippocampus to mPFC) has been observed in retrieval of episodic memory. In a recent study, Place et al1investigated the functional dynamics of hippocampal and prefrontal interactions in rats as they used spatial context to guide retrieval of object memories.
Rats were first trained on the context-guided memory task to associate reward with 1 of the 2 items in each of the distinctive environments or contexts. The rats first explored a context in the “context exploration period.” This was followed by an “object sampling” period in which rats were presented with the 2 objects, eventually selecting an object by attempting to retrieve a reward within it (Figure, A). Local field potentials were recorded in the hippocampus and mPFC throughout the trial. During the initial second of both the context exploration and object sampling periods, maximal coherence between hippocampal and mPFC local field potentials was observed in the theta frequency range (7-12 Hz). In addition, single units with task-relevant activity exhibited spiking that was phase-locked to local theta oscillations. These findings indicate that local theta activity detected in the mPFC and hippocampus reflects neural activity in these areas relevant to context exploration and object sampling and furthermore that the hippocampus and mPFC may be interacting during these tasks through theta frequency oscillatory activity.
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