Humans have an innate ability to deal with numerosity and other aspects of magnitude. This ability is generally honed through education in and experience with mathematics, which necessarily changes the brain structurally and functionally. These changes can be further manipulated through non-invasive electrical brain stimulation. Studying these processes in the case of maths not only constitutes research of great practical impact – given the importance of numerical skills in today's society – but also makes use of maths as a suitable domain in which to study plasticity. In this thesis, I aimed to explore how expertise with numbers shapes brain and behaviour, and also the degree to which processing numbers is similar to other domains in terms of the necessity of healthy brain regions believed to underlie normal processing within and across these domains. In Study 1, behavioural and structural brain differences were found cross-sectionally between mathematicians and non-mathematicians. A double dissociation between those groups was found between grey matter density in the frontal lobe and behavioural performance: their correlation was positive for mathematicians but negative for controls. These effects may have been caused by years of experience, by congenital predispositions, or, plausibly, by both of these factors, whose disambiguation is non-trivial. Study 2 used transcranial random noise stimulation (tRNS) to assist arithmetic learning. A novel montage was used to enhance brain function during the stage when it is believed to be most involved. Real as compared to sham tRNS enhanced reaction times (RTs) and learning rate on a calculation-based task, but not on a retrieval-based task. The effects were only observed in conditions of high task difficulty. Study 3 examined structural MRI measures before and after arithmetic training to determine how either frontal or parietal tRNS applied with the task changes the structure of the brain longitudinally as compared to sham. Previous results (including those of Study 2) of behavioural facilitation in terms of enhanced RTs to calculation problems were replicated, and further interpreted. Both frontal and parietal tRNS modulated the changes that occurred, pre-to-post training, in terms of cortical volume and gyrification of frontal, parietal and temporal areas. Study 4 investigated the shared neural and cognitive resources used for processing numerical magnitude and musical pitch, by probing how stimulus-response compatibility (SRC) effects for each of the two dimensions compare in a group of mainly temporoparietal lesion patients with numerical impairments versus controls. A double dissociation was found in that numerically impaired patients did not show the number-based SRC effect but did show the pitch-based one, while control subjects demonstrated the opposite trend. Overall, the results of these studies leave us with three main messages. First, expertise in numbers and mathematics, whether acquired through years of experience (Study 1) or through a few days of tRNS-assisted training (Study 3), appears to be associated with complex changes in the morphology of several brain structures. Some – but not all – of these structures are maths-relevant, and, in the case of tRNS-assisted training, they are distal to the site of the stimulating electrodes. Second, tRNS can improve performance in arithmetic (Studies 2 and 3), although the mechanisms by which this occurs are not yet fully understood, neither neurally nor behaviourally. Third, I found (Study 4) that brain lesions leading to impairment in the number domain do not necessarily affect processing in other domains – such as pitch – that are otherwise linked to number via a putative common code in the parietal lobes.

L’objectif principal de cette thèse est d’évaluer l’impact thérapeutique d’une intervention de stimulation cognitive associée à un programme d’activité physique au stade léger à modéré de la maladie d’Alzheimer. Précisément, il s’agit de détailler les effets respectifs et combinés de ces interventions selon les trois dimensions suivantes : 1) le fonctionnement cognitif général, 2) le fonctionnement exécutif à travers la mise à jour, l’alternance, l’inhibition et la planification, 3) le fonctionnement psychosocial, intégrant l’anxiété et la dépression. L’étude principale de cette thèse a comparé les performances pré- et post-intervention de 67 patients répartis en quatre groupes (i.e., groupe stimulation cognitive, groupe activité physique, groupe stimulation cognitive plus activité physique, groupe contrôle). Les résultats ont montré que, 1) la stimulation cognitive permet l’amélioration du fonctionnement cognitif général et le maintien des capacités d’alternance et de mise à jour, 2) l’activité physique favorise l’amélioration des capacités d’inhibition et le maintien des capacités d’alternance, et 3), l’association de la stimulation cognitive et de l’activité physique permet l’amélioration du fonctionnement cognitif général, l’amélioration des performances d’alternance et d’inhibition et le maintien des capacités de mise à jour. L’ensemble de ces résultats met en évidence le potentiel thérapeutique de la stimulation cognitive et de l’activité physique dans le traitement de la maladie d’Alzheimer. De plus, les données recueillies montrent que les bénéfices des interventions sont d’autant plus importants lorsque le programme intègre les deux formes de prises en charge. Les mécanismes par lesquels s’opèrent les changements dans la cognition des personnes souffrant d’Alzheimer sont discutés à travers le concept de réserve cognitive. L’enjeu des futures recherches réside dans le développement de thérapies visant le maintien du fonctionnement exécutif à travers la stimulation des capacités physiques et cognitives. / The principal objective of this thesis was to evaluate the therapeutic impact of an intervention which combines a cognitive stimulation program and a physical activity program in mild to moderate Alzheimer's disease. Specifically, the objective was to detail the respective and combined effects of these interventions in the three following dimensions, 1) general cognitive functioning, 2) executive functioning with updating, switching, inhibition and planning, 3) psychosocial functioning, including anxiety and depression. The main study of this thesis examines the pre-and post-intervention scores of 67 patients assigned into four groups (i.e., cognitive stimulation, physical activity, cognitive stimulation combined with physical activity, and control). The data showed that, 1) cognitive stimulation improved general cognitive functioning and maintained updating and switching abilities, 2) physical activity improved inhibition and switching abilities, and 3) associate cognitive stimulation and physical activity allowed the improvement of general cognitive functioning, inhibition and switching abilities, and allowed remaining updating abilities. Taken together, these results demonstrate the therapeutic effects of cognitive stimulation and physical activity in Alzheimer's disease treatment. Moreover, the data collected showed that the benefits of the interventions were particularly important when the program included both interventions. Mechanisms of changes in cognition of Alzheimer's patients are discussed through the concept of cognitive reserve. The aim for future researches is to develop programs of cognitive stimulation and physical activities to preserve executive functioning.

L’objectif général de ce travail de thèse était d’étudier la possibilité d’améliorer le fonctionnement cognitif (mémoire de travail, mémoire épisodique, vitesse de traitement, fonctions exécutives) et la santé psychologique (estime de soi, plainte mnésique, satisfaction de vie) lors du vieillissement grâce à la pratique des activités de loisir. Pour cela, quatre programmes utilisant les activités de loisir, allant du plus unimodal au plus multimodal, ont été conçus et proposés à des participants âgés. Leurs scores à différentes mesures ont été évalués avant et après les interventions et comparés à ceux de groupes contrôles. Les résultats mettent en évidence que globalement ces programmes permettaient l’amélioration des performances cognitives ainsi que des transferts sur la santé psychologique, sauf l’entraînement unimodal (i.e., spécifique). / The main objective of this thesis was to test the possibility to improve cognitive functioning (working memory, episodic memory, speed of processing, executive functions) and psychological health (self-esteem, memory complaint, satisfaction of life) with leisure activities during aging. The present work was based on four programs using leisure activities, from unimodal to multimodal composition including pretests and posttests and compared to control groups. Globally, results show that the programs heterogeneously improve cognitive performances and provide evidence of benefits transfer to psychological health, except for the unimodal program. Overall, results show that unimodal cognitive training only improve cognitive functioning (Experience 1), whereas playful and multimodal interventions improve psychological health and influence cognitive performance (Experiences 2, 3, et 4). Eventually, our results show that leisure activities during lifespan are associated with a better psychological health (Experiment 3), improve subjective well-being and help to better adapt to ageing. This work supports the idea that leisure activities during life span are in line with psychological health. It also highlights that multimodal cognitive interventions contribute to successful ageing more than unimodal programs.