The cell division and differentiation, assisted by cell-cell interactions, doesn't just occur in the ventricular zone. Cell division and differentiation can also occur in adulthood, particularly in the dentate gyrus of the hippocampus and the olfactory bulbs. Together with synaptic rearrangement and cell death, neurogenesis is a contributor to plasticity. There is evidence to suggest that learning and exercise enhance adult neurogenesis, although the neurons formed in this process still constitute the minority of neurons in the brain.