There is common agreement that experience plays a critical role in shaping the organization of the brain. We have seen that a number of brain systems show sensitivity to experience only during limited time periods, called sensitive periods. This is a time period during which experience may profoundly and durably alter the organization of that system. In contrast, other systems display life-long sensitivity to experience. There is ample evidence that similar molecular and cellular mechanisms are at play to mediate these different kinds of plastic changes. Why then does plasticity during sensitive periods appear to be more pervasive than at other stages of life? Though there is no clear answer to that question, a few hints are available. First, there appear to be molecular and cellular differences between the immature and later stages of life. For example, the NMDA receptor that mediates learning in adults is also found in young animals but in a form that facilitates learning and plastic changes. The NMDA receptor in young animals is triggered more easily and leads to larger neuronal responses than in adults; these properties suggest that learning is likely to happen more readily in young animals. It has also been proposed that these properties may be especially suited to maintain synapses in regions that do not receive well-correlated inputs, in effect keeping the multipotentiality of the young cortex. This view is supported by computational models of development, which illustrate that the redundancy of the connectivity at the onset of sensitive periods may allow for easier learning and adaptation than what can be achieved once the architecture of the brain has committed to a given pattern of organization. Thus, even if some of the mechanisms are similar or even identical between developmental and adult plasticity, the fact that these mechanisms operate on nervous systems that are structurally and physiologically different may account for the observation that plastic changes in development often become irreversible, whereas plastic changes in adults often are reversible provided adequate training.

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