Neuroplasticity (also referred to as brain plasticity, cortical plasticity or cortical re-mapping) is the changing of neurons and the organization of their networks and so their function by experience. This idea was first proposed in 1892 by Santiago Ramón y Cajal the proposer of the neuron doctrine though the idea was largely neglected for the next fifty years. The first person to use the term neural plasticity appears to have been the Polish neuroscientist Jerzy Konorski.

The brain consists of nerve cells or neurons (and glial cells) which are interconnected, and learning may happen through changing of the strength of the connections between neurons, by adding or removing connections, or by adding new cells. "Plasticity" relates to learning by adding or removing connections, or adding cells. During the 20th century, the consensus was that lower brain and neocortical areas were immutable in structure after childhood, meaning learning only happens by changing of connection strength, whereas areas related to memory formation, such as the hippocampus and dentate gyrus, where new neurons continue to be produced into adulthood, were highly plastic.

This belief is being challenged by new findings, suggesting all areas of the brain are plastic even after childhood. Hubel and Wiesel had demonstrated that ocular dominance columns in the lowest neocortical visual area, V1, were largely immutable after the critical period in development. Critical periods also were studied with respect to language; the resulting data suggested that sensory pathways were fixed after the critical period.

However, studies determined that environmental changes could alter behavior and cognition by modifying connections between existing neurons and via neurogenesis in the hippocampus and other parts of the brain, including the cerebellum. Decades of research have now shown that substantial changes occur in the lowest neocortical processing areas, and that these changes can profoundly alter the pattern of neuronal activation in response to experience. According to the theory of neuroplasticity, thinking, learning, and acting actually change both the brain's physical structure (anatomy) and functional organization (physiology) from top to bottom.

Neuroscientists are presently engaged in a reconciliation of critical period studies demonstrating the immutability of the brain after development with the new findings on neuroplasticity, which reveal the mutability of both structural and functional aspects. A substantial paradigm shift is now under way: Canadian psychiatrist Norman Doidge has in fact stated that neuroplasticity is "one of the most extraordinary discoveries of the twentieth century."

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