Memory representation in the medial prefrontal cortex Cortical communication, and the development of a prefrontal ensemble code for associative memory

Despite knowledge of the critical importance of the medial prefrontal cortex (mPFC) in both the consolidation and long-term retrieval of event memory in the brain, it is unclear which aspects of a memory are represented within this region and how they change with learning and memory consolidation. Likewise, how the mPFC interacts with other facets of the memory network to shape these representations is also uncertain. In this thesis I employed electrophysiological techniques to examine individual prelimbic mPFC (PrL) neurons selectivity for various task features in an associative memory paradigm in rats across learning, consolidation and over-training. Further, I recorded oscillatory activity from the rhinal cortices and ventral hippocampus to examine how these regions may modulate mPFC memory representations across time. I reveal similar proportions at all time-points of both neurons highly selective for specific task features, and neurons exhibiting more generalized activity. Looking at population level activity across time however revealed a shift from more discrete encoding during learning to more generalized encoding post-consolidation. Further, PrL neurons whose firing patterns were modulated by rhinal and ventral hippocampal theta oscillations uncover a potential source of this change in encoding across time. Specifically, I show that discrete populations of PrL neurons display phase-locked spiking to theta oscillations in the lateral entorhinal cortex, the perirhinal cortex, and ventral hippocampus. Whereas the proportions of these different populations were not observed to change across time, the information contained within their activity did. PrL neurons phase-locked to rhinal theta oscillations were highly selective to task features early on in the acquisition of the memory and became more generalized with learning and consolidation. Neurons phase-locked to ventral hippocampal theta on the other hand became more selective for the meaning of the stimulus post-consolidation. I discuss how these findings fit with existing theories of the role of the mPFC in long-term memory including schema development and indexing of cortical memory activity. Taken together, these results demonstrate memory encoding patterns within the prelimbic mPFC and highlight, through communication with several important nodes of the memory network, potential mechanisms through which the encoding of different memory features evolve across learning.