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[Brain plasticity during development: physiological bases and functional MRI approach].

Brain functional MRI (fMRI) is a new tool for the study of the development of cognitive functions in healthy children ("natural plasticity"), as well as for the assessment of functional reorganization following brain lesions. However, methodological difficulties related to the pediatric population (movements, cooperation), along with unsolved issues about the influence of physiological parameters of the immature brain on fMRI results, explain the limited number of published studies. Normal brain maturation is characterized by a transient phase of synaptic redundancy followed by selective synaptic regression until adulthood, that forms the neurobiological correlates of both learning and individual variability of cortical anatomy and functional organization, and of the large potential for post-lesional plasticity in children. fMRI in school-age children demonstrated activation patterns comparable to adults during motor, language, and working memory tasks. In neonates and infants, fMRI showed significant differences of visual cortex activation. Post-lesional plasticity is more pronounced in younger children. In motor cortex, activation of ipsilateral hemisphere may be seen in cases of rolandic lesions. Interhemispheric shift of language networks occurs mostly in cases of destructive or large brain lesions, or in cases of early refractory epilepsy.