If theta phase-locking between hippocampus and mPFC is necessary for certain memory operations, as suggested by the studies described above, then the development of theta phase-locking between the regions should parallel the acquisition of learned behavior. A recent study addressed this hypothesis by simultaneously recording from ventral or intermediate CA1 and mPFC in rats that learned reward contingency rules on a Y-maze [19••]. Theta coherence between mPFC and CA1 increased at the decision point of the maze, and this increase was significantly greater after animals had successfully learned the rule associated with reward. Moreover, a higher percentage of mPFC neurons fired together on the same phase of hippocampal theta after learning. This enhancement of correlated firing may enable mPFC to more effectively activate its outputs after learning has occurred. Remarkably, the authors observed similar effects (i.e. increased theta coherence and synchrony of mPFC neurons) when dopamine was applied in mPFC, raising the possibility that learning-induced increases in theta synchrony are mediated by dopaminergic inputs to mPFC that signal expectation of reward [27].
