(a) According to theories based on the classic trisynaptic loop[23, 25, 88], the DG is the principal input to the hippocampus, positioning neurogenesis for a significant impact on memory formation. (b) An alternative possibility envisions that direct connections from EC to CA1 are sufficient for some hippocampal processing and loops through the CA3 and the DG are important under specific circumstances, such as one-shot learning[32, 89]. This positioning would suggest that DG modulates CA3 function and neurogenesis modulates this modulation.