The dentate gyrus is one subdivision of the mammalian hippocampus which has no clear correspondence in birds or reptiles, despite any superficial homology. It is characterized by sparse and powerful unidirectional projections to CA3 pyramidal cells, the so-called mossy fibers. Mossy fiber synapses appear to duplicate, in terms of the information they convey, what CA3 cells already receive from entorhinal cortex layer II cells, which project both to the dentate gyrus and to CA3. It is thus at the same time the most striking component, within the structure in our brain which is critical for memory formation, and the component most difficult to understand. David Marr, in his theory for archicortex, gives up trying to understand the dentate gyrus. Yet, the intellectual trajectory trailblazed by Marr’s ideas has led, decades later, to make sense of the dentate gyrus, and to appreciate, almost by absurdum, the power of Marr’s intuition, which was way more advanced than the tools, mathematical and empirical, which he had at his disposal
