Coordinating what we've learned about memory consolidation: Revisiting a unified theory

According to traditional systems consolidation theories neocortical long-term plasticity (i.e., cellular consolidation) lags behind, and is dependent upon, hippocampal long-term plasticity. In this review, we examine accumulating evidence that local neocortical and hippocampal cellular consolidation occurs with a similar time-course. The implication is that the rate-limiting step for systems consolidation is the time it takes for cellular consolidation in longer connections throughout a more distributed extra-hippocampal system that comes to coordinate distributed neocortical activity during recall. The hippocampus is, thus, crucial for the development of this extra-hippocampal coordinating system, and acts to coordinate activities crucial for recall until it develops. Recent work on schema formation, engram cells, and the role of sleep in consolidation add substantial evidence for this "unified theory" of systems and cellular consolidation. Here, we discuss this evidence, its implications, and consider remaining questions.