Neural Ensembles and Local Field Potentials in the Hippocampal-Prefrontal Cortex System During Spatial Learning and Strategy Shifts in Rats

To better understand the mechanisms of hippocampal-cortical signalling for mnemonic function and behavior, we recorded ventral hippocampal local field potentials (LFPs) simultaneously with ensemble neural activity and LFPs in the prefrontal cortex in rats learning and switching strategies in a Y-maze variant of the Wisconsin Card Sorting Task. Behaviorally correlated activity of individual neurons changed after task contingency changes or as the animal adopted different strategies. Curiously, within ensembles of simultaneously recorded neurons, individual neurons showed different responses to contingency or strategy while others did not change at all. Furthermore some prefrontal neurons are significantly modulated by hippocampal theta activity and also by hippocampal sharp waves (with 150 Hz ripples). Finally we present data showing that coherence between the hippocampus and prefrontal cortex in the theta band is behaviorally modulated. These observations provide evidence for engagement of hippocampal signals by prefrontal neurons as a mechanism underlying learning this task.