Neuroplasticity, also known as brain plasticity or neuroplasticity, is the ability of the brain to change throughout an individual's life, e.g., brain activity associated with a given function can be transferred to a different location, the proportion of grey matter can change, and synapses may strengthen or weaken over time. Research in the latter half of the 20th century showed that many aspects of the brain can be altered (or are "plastic") even through adulthood. However, the developing brain exhibits a higher degree of plasticity than the adult brain.

Neuroplasticity can be observed at multiple scales, from microscopic changes in individual neurons to larger-scale changes such as cortical remapping in response to injury. Behavior, environmental stimuli, thought, and emotions may also cause neuroplastic change through activity-dependent plasticity, which has significant implications for healthy development, learning, memory, and recovery from brain damage. At the single cell level, synaptic plasticity refers to changes in the connections between neurons, whereas non-synaptic plasticity refers to changes in their intrinsic excitability.

One of the fundamental principles underlying neuroplasticity is based on the idea that individual synaptic connections are constantly being removed or recreated, largely dependent upon the activity of the neurons that bear them. The activity-dependence of synaptic plasticity is captured in the aphorism which is often used to summarize Hebbian theory: "neurons that fire together, wire together"/"neurons that fire out of sync, fail to link". If two nearby neurons often produce an impulse in close temporal proximity, their functional properties may converge. Conversely, neurons that are not regularly activated simultaneously may be less likely to functionally converge.

Cortical organization, especially in sensory systems, is often described in terms of maps. For example, sensory information from the foot projects to one cortical site and the projections from the hand target another site. As a result, the cortical representation of sensory inputs from the body resembles a somatotopic map, often described as the sensory homunculus.

The adult brain is not entirely "hard-wired" with fixed neuronal circuits. There are many instances of cortical and subcortical rewiring of neuronal circuits in response to training as well as in response to injury. There is solid evidence that neurogenesis (birth of brain cells) occurs in the adult, mammalian brain—and such changes can persist well into old age. The evidence for neurogenesis is mainly restricted to the hippocampus and olfactory bulb, but current research has revealed that other parts of the brain, including the cerebellum, may be involved as well. However, the degree of rewiring induced by the integration of new neurons in the established circuits is not known, and such rewiring may well be functionally redundant.

A surprising consequence of neuroplasticity is that the brain activity associated with a given function can be transferred to a different location; this can result from normal experience and also occurs in the process of recovery from brain injury. Neuroplasticity is the fundamental issue that supports the scientific basis for treatment of acquired brain injury with goal-directed experiential therapeutic programs in the context of rehabilitation approaches to the functional consequences of the injury.

Several companies have offered so-called cognitive training software programs for various purposes that claim to work via neuroplasticity; one example is Fast ForWord which is marketed to help children with learning disabilities. A systematic meta-analytic review found that "There is no evidence from the analysis carried out that Fast ForWord is effective as a treatment for children's oral language or reading difficulties". A 2016 review found very little evidence supporting any of the claims of Fast ForWord and other commercial products, as their task-specific effects fail to generalise to other tasks.

Neuroplasticity is involved in the development of sensory function. The brain is born immature and it adapts to sensory inputs after birth. In the auditory system, congenital hearing impairment, a rather frequent inborn condition affecting 1 of 1000 newborns, has been shown to affect auditory development, and implantation of a sensory prostheses activating the auditory system has prevented the deficits and induced functional maturation of the auditory system. Due to a sensitive period for plasticity, there is also a sensitive period for such intervention within the first 2–4 years of life. Consequently, in prelingually deaf children, early cochlear implantation, as a rule, allows the children to learn the mother language and acquire acoustic communication.

In the phenomenon of phantom limb sensation, a person continues to feel pain or sensation within a part of their body that has been amputated. This is strangely common, occurring in 60–80% of amputees. An explanation for this is based on the concept of neuroplasticity, as the cortical maps of the removed limbs are believed to have become engaged with the area around them in the postcentral gyrus. This results in activity within the surrounding area of the cortex being misinterpreted by the area of the cortex formerly responsible for the amputated limb.

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