The primary mechanisms of neuroplasticity relevant to learning are the strengthening and weakening of existing synaptic connections, not neurogenesis.

“the change that underlies neuroplasticity at the level of cells which we call neurons or nerve cells generally involves three different mechanisms one is the strengthening of certain connections what we call synaptic connections... another form of neuroplasticity is the weakening of connections between neurons and yet a third form of plasticity which is often discussed in the media but is very rare actually in the nervous system especially the adult nervous system of humans is neurogenesis or the addition of new neurons”