Substrats neuronaux du traitement visuel et sémantique des mots dans le vieillissement normal : apports de la MEG

While it has long been assumed that the organization of the brain network underlying semantic processing remains intact in normal aging, mainly due to older adults’ intact behavioral performance on semantic tasks, several recent studies suggest that brain changes underlying semantic processing operate during aging. These changes appear to affect mainly the brain regions responsible for the executive aspects of semantic memory (SM), involved in semantic search and selection processes, as well as the strategic manipulation of semantic knowledge. However, the specific mechanisms underlying cerebral reorganization of semantic processing in normal aging are not well understood, partly because of methodological differences among studies. Recent literature also suggests that brain changes may be observed in relation to visual perceptual aspects of word processing in older adults. Since reading words is a dynamic interactive process between low-level perceptual functions and higher-order processes such as semantic processing, there may be age-related changes in terms of brain interactions between perceptual and semantic aspects of word processing. The general aim of this thesis was to characterize the cortical changes and the time course of brain signal associated with semantic and perceptual processing of words, as well as the modulations between semantic and perceptual processes in normal aging, using magnetoencephalography (MEG) as the investigative method.  Firstly (Chapter 2), the patterns of brain activation of two groups of healthy younger and older adults were compared relative to a semantic task participants carried out during MEG acquisition, by focusing on the signal around the N400, a component associated with semantic processing. The results indicate that brain changes associated with normal aging mainly affect structures involved in the executive aspects of semantic processing. Greater activation was observed in prefrontal cortex for younger relative to older adults, while the latter group of participants activated the temporoparietal region to a greater extent than young adults. Moreover, the left anterior temporal lobe (ATL), considered to be a central and amodal region of semantic processing, was also more activated by older than younger participants.   Secondly (Chapter 3), specific patterns of brain activation of younger and healthy older adults were compared in relation to visual perceptual processing, by focusing on the 200 first milliseconds of cortical signal during word processing. The results show that the age-related brain changes affect the fusiform gyrus, as well as the semantic network, with greater activation found in these regions in the group of older participants relative to younger participants, while no difference in activation of the visual extrastriate cortex was found between groups. The theoretical implications of the results of these two studies are discussed. Finally, limitations of this thesis and future perspectives are addressed (Chapter 4).

Keyphrases: normal aging, language, semantic memory, magnetoencephalography, visual perception, word recognition