Brain plasticity: a new concept in neuroscience, a new tool in neurosurgery
At the beginning of the 19th century, two opposite conceptions of the functioning of the central nervous system were suggested. One was the theory of “equipotentiality”, which hypothesized that the entire brain, or at least one complete hemisphere, was im
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Introduction At the beginning of the 19th century, two opposite conceptions of the functioning of the central nervous system were suggested. One was the theory of “equipotentiality”, which hypothesized that the entire brain, or at least one complete hemisphere, was implied in the practice of a functional task. The other was the theory of “localizationism”, which supposed each part of the brain to correspond to a specific function and was built following the seminal description of “phrenology”. Progressively, the frequent reports of lesional studies led to an intermediate view, namely, a brain organized (i) in highly specialized functional areas, called “eloquent” regions (such as the central, Broca’ s, and Wernicke’ s areas, early identified), for which any lesion gives rise to major irrevocable neurological deficits, and (ii) in “nonfunctional” structures, whose lesions would be without clinical consequences. On the basis of these first anatomo-functional correlations and despite some pioneer observations of postlesional recovery, the dogma of a static functional organization of the brain, which would not be able to compensate any injury involving the so-called eloquent areas, was settled for a long time. However, through regular reports of improvement of the functional status after damage to cortical and/or subcortical structures considered as “critical”, H. Duffau (ed.), Brain Mapping © Springer-Verlag/Wien 2011
this view of a “fixed” central nervous system was called into question in the past decades. Consequently, many investigations were performed, initially in vitro and in animals and more recently, since the development of brain mapping, in humans, in order to study the mechanisms underlying these compensatory phenomena, and the concept of cerebral plasticity was born (for a review, see [4]).
Brain plasticity: definitions and mechanisms Cerebral plasticity can be defined as the continuous processing allowing short-, medium-, and long-term remodelling of the neuronalsynaptic organization, in order to optimize the functioning of the networks of the brain during phylogeny, ontogeny, and physiological learning and after lesions involving the peripheral or the central nervous system. Several hypotheses about the pathophysiological mechanisms underlying plasticity have been considered. At a microscopic scale, these mechanisms seem to be essentially represented by synaptic efficacy modulations, unmasking of latent connections, phenotypic modifications, synchrony changes, and neurogenesis. At a macroscopic scale, diaschisis, functional redundancies, crossmodal plasticity with sensory substitution, and morphological changes are suggested to be in-
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volved. Moreover, the behavioral consequences of such cerebral phenomena have been analyzed for humans in the past decade, both in physiologic – ontogeny and learning – and in pathologic conditions. In particular, the ability to recover after a lesion of the nervous system and the patterns of functional reorganization within eloquent areas and/or within
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