Functional Relevance of Cortical Plasticity

Summary

Different reports have demonstrated cortical plasticity associated with large hemispheric lesions, skill acquisition and use. However, it is not clear if this plasticity is always useful in terms of compensatory function. A patient who had a hemispherectomy during childhood is presented as an example of functionally relevant plasticity in the motor domain. At the time of testing, he was able to voluntarily move the affected arm. Transcranial stimulation demonstrated the development of different cortical representations for both arms in the remaining hemisphere. Therefore, the remaining hemisphere took over some of the functions of the missing hemisphere. Functional relevance of cortical plasticity can be also demonstrated when reorganization takes place across sensory modalities. Cortical areas normally processing visual information can be activated by somatosensory input. To determine if this activation contributes to behavioral performance in tactile discrimination tasks, repetitive transcranial magnetic stimulation was delivered to different scalp positions during reading of Braille and embossed Roman letters by blind and sighted subjects. Stimulation of the occipital areas disrupted reading and induced phantom tactile sensations in the early blind, but not in sighted volunteers. The occipital cortex is not only active in association with, but is one of the important functional components of the network mediating tactile discrimination in the blind. Determination of the functional role played by plasticity in each setting is important for the establishment of rational strategies for promoting recovery of function in humans.