Experience-Dependent Plasticity

Experience-dependent plasticity refers to changes in the anatomical or functional organization of the nervous system as a consequence of sensory input or motor output. This can be at the anatomical level of how different neurons project from one area to the next (e.g., how neurons from the eye [Page 408]send their axons to the brain). Alternatively, the gross anatomical projections themselves may not change; rather, the individual synapses are refined, or pruned, so that only restricted recipient neurons are contacted. Finally, there can be instances where functional changes occur in the absence of anatomical changes. In this case, certain synapses are strengthened, whereas other synapses are weakened, with the result that neurons that used to respond to a particular sensory input now respond to a different sensory input (e.g., from touching the index finger to touching the thumb).

Plasticity occurs under a number of different conditions, such as development, neuronal injury, or experience. For example, the amputation of a finger will result in a reorganization of the primary somatosensory cortex so that neurons that used to respond to the amputated finger come to represent the adjacent, intact finger. A second example is following massive injury to a particular sensory structure, such as the eyes leading to blindness or to the cochlea leading to deafness. In these cases, evidence indicates that the parts of the brain that usually represent the missing sensation come to respond to a different sensation, such as visual areas responding to auditory stimuli in blind individuals and auditory areas responding to visual input in deaf individuals.

Plasticity strongly depends on the stimulus history of the individual. This can be the result of experience during development or in adulthood well after all developmental processes have been completed. This entry provides a brief description of both developmental and adult plasticity, providing examples of these plastic changes in different sensory systems.