Controlling the critical period.

Neuronal circuits are shaped by experience during 'critical periods' of early postnatal life. The ability to control the timing, duration, and closure of these heightened levels of brain plasticity has recently become experimentally possible. Two seemingly opposed views of critical period mechanism have emerged: (1) plasticity may be functionally accessed throughout life by appropriately modified stimulation protocols, or (2) plasticity is rigidly limited to early postnatal life by structural modifications. This overview synthesizes both perspectives across a variety of brain regions and species. A deeper understanding of critical periods will form the basis for novel international efforts to "nurture the brain".