Impact of preventive uni or multidomain interventions on functional connectivity and white matter microstructure for participants at-risk to develop Alzheimer Disease. ; Impact d’interventions préventives uni ou multidomaines sur la connectivité fonctionnelle et la microstructure de la substance blanche chez des personnes à risque de développer la maladie d’Alzheimer.

Alzheimer's disease is the most common cause of dementia. Although numerous therapeutic trials aimed at preventing its onset have been conducted or are still underway, no cure for the disease is available at this time. The difficulties encountered in establishing a treatment have led to the consideration of other approaches in the fight against Alzheimer's disease. It has been shown that the disease is associated with several risk factors. Some of these factors, such as physical inactivity or smoking, are environmental factors that can be modified. In response to this finding, several clinical trials have implemented preventive strategies to slow cognitive decline and decrease the incidence of the disease. Although some interventions show a beneficial effect, questions remain as to the optimal type of intervention to apply, the different parameters maximizing the effect of the intervention, the type of person to target and the mechanisms that result from the interventions. Recently, the use of brain imaging and MRI has allowed for a more detailed characterization of how these preventive interventions act on the brain.The objective of this thesis was to evaluate the effect of uni or multi-domain interventions, targeting several risk factors of the disease, on the brain of elderly people at risk for Alzheimer's disease. The effect of interventions on the evolution of functional connectivity was studied in a first step. In a second step, the effect of the interventions on the integrity of the white matter was evaluated.The work carried out during this thesis did not show any effect of the multidomain interventions on brain functional markers or on the integrity of the white matter. However, it is observed that the effect of omega-3 supplementation on functional connectivity depends on the cognitive status of the participants examined. This indicates that it may be necessary to stratify participants in intervention studies into groups with homogeneous clinical profiles in order to accurately characterize the effect ...