The brain processes information and emotions in neuronal networks that gradually develop under genetic and environmental guidance during early postnatal life. Neuronal networks are fine-tuned through neuronal plasticity, whereby those synapses that optimally mediate the presented environmental information are selected and strenghtened while the connections that mediate random noise are weakened and pruned away. Changes in the networks take place readily during the critical developmental periods of early life, but after these critical periods plasticity is reduced and restricted.
The mammalian visual cortex has become a well-characterized model system for neuronal network plasticity. During mammalian postnatal development, visual inputs from each eye segregate into eye-specific regions in the primary visual cortex, called ocular dominance (OD) columns. This segregation process takes place during a critical period of plasticity during early postnatal development and requires the balanced use of both eyes. If vision of one eye is prevented during the critical period, the more active inputs from the open eye take over the majority of the visual cortex and the weaker eye looses its connectivity, thereby becoming permanently poor in vision, or amblyopic. In an adult animal, after the closure of the critical period, the connectivity becomes stabilized and the patching of one eye no longer influences the OD distribution; therefore, after the end of the critical period (in humans, between the age of 7-10 years), amblyopic eye becomes permanently weak in vision.
This model system has been utilized extensively to reveal the underlying mechanisms of the plastic process whereby the visual experience, through the activity of retinal projections, gradually shapes neuronal networks within the visual cortex to optimally represent the visual environment. It is widely considered that similar processes govern the development and tuning of neuronal connectivity in other cortical areas as well.