JLE

Revue de neuropsychologie

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The adaptative brain: the contribution of hemispheric coupling to the preservation of cognitive abilities with age and its uncoupling in Alzheimer’s disease Volume 1, issue 2, juin 2009

Authors
Centre de Recherche, Institut Universitaire de Gériatrie de Montréal et Faculté de médecine, Université de Montréal, Québec, Canada, Laboratoire de Psychologie Cognitive et Sociale EA 1189, Unité de Formation et de Recherche Lettres Arts et Sciences Humaines, Pôle universitaire Saint-Jean-d’Angély, Université de Nice-Sophia-Antipolis, Nice

Essentially supported by the corpus callosum, the interaction between the two hemispheres represents a flexible mechanism which allows both hemispheres to be coupled in the presence of more complex cognitive tasks. This adaptative mechanism allows the optimal distribution of cognitive processing between the two hemispheres. Thus, the division of processing across the hemispheres would provide more computational power than sustaining processing within a single hemisphere, since a larger number of independent brains regions are recruited. Furthermore, the increase in computational power allowed by interhemispheric cooperation could be especially beneficial for the more complex tasks, those requiring a higher number of computations. In high demanding conditions, the benefits associated with greater computational power become more substantial than the costs incurred by the interhemispherical processing, leading to an across-hemisphere advantage. In this review, the contribution or non-contribution of this adaptative callosal mechanism is discussed by reference to successful aging and to Alzheimer’s disease. In successful aging, it is suggested that this mechanism represents the back-bone of the inter-hemispheric reallocation of brain activation as reported in many neuroimaging studies. Consistent with the hypothesis it is suggested that greater hemispheric coupling in older adults contributes to counteract age-induced neurocognitive deficits through the increased involvement of both hemispheres. However, in the case of Alzheimer’s disease, and despite the increased necessity of this mechanism in order to face cognitive limitations, inter-hemispheric coupling is thought not to occur because of the concomitant callosal atrophy which is present in this disease. Inter-hemispheric coupling is thus a dimension of the brain’s functional optimal functioning, at least at the cognitive level.