Attention-Deficit/Hyperactivity Disorder (ADHD) and Autism Spectrum Disorder (ASD) are two neurodevelopmental disorders with distinct diagnostic criteria that often co-occur. Although both ASD and ADHD are associated with structural and functional brain abnormalities when compared to typically developing (TD) populations, it is necessary to disentangle the shared and specific abnormalities between these disorders, potentially underlying similarities and differences in their clinical and neurocognitive profiles. The aim of this thesis was to explore the shared and disorder specific functional and structural brain abnormalities between ASD and ADHD. For this purpose, the neural underpinnings of a group of children with ADHD, a group of children with ASD and a group of TD children aged 8 to 12 years old were compared using different neuroimaging techniques. In Chapter 2, the experimental sample included in this thesis was described using multiple clinical and neurocognitive measures. In the first study (Chapter 3), total and regional brain volumes were compared between groups, using voxel-based morphometry. The results of this study showed larger grey matter volume (GMV) in the left precuneus and decreased GMV in the right thalamus in the ADHD group compared to either the TD or the ASD groups, and increased GMV in the right precentral gyrus in the ASD group compared to either the ADHD or the TD groups. In the second study (Chapter 4), white matter microstructure was compared between groups using diffusion tensor imaging derived indices (fractional anisotropy [FA] and mean diffusivity [MD]). Reduced FA (i.e. reduced diffusion directionality) in the genu of the corpus callosum (CC) was found in the ASD group compared to either children with ADHD or TD children, whereas lower FA in the body of the CC was a shared feature between the ADHD and ASD groups. Finally, in the last study (Chapter 5), inhibition-related brain activation was compared between groups during the execution of an inhibition stop-signal task. In children with ADHD, successful inhibition was associated with right inferior parietal activation, whereas right frontal regions were activated in children with ASD. Between-group comparisons disclosed higher middle frontal activation in the ASD group compared to the ADHD or the TD groups. Taken together, our findings provide further evidence contributing to disentangle the shared and specific brain structural and functional abnormalities between ASD and ADHD. / Doctorat en Sciences psychologiques et de l'éducation / info:eu-repo/semantics/nonPublished

This work deals with the different manifestations of cooperativity in pedopsychiatry in connection with severe disorders in adolescence such as suicide attempts and runaways, and developmental processes in early childhood. Our principal hypothesis is that cooperativity, nonlinearity and the resulting feedback loops underlie the appearance of suicidal peaks, the transitions between different mental states in a population of susceptible, suicidal and runaway individuals, or finally the selfish to egalitarian transition in young children. The principal methodology put forward is mathematical modeling, which allows one to identify the principal mechanisms present, to assess the role of the environmental constraints and to make predictions. This provides in turn insights on possible prevention, managemant and therapy strategies. Using this approach we have shown that suicide contagion can take an explosive form that is dramatically accelerated by the presence of virtual contacts as it happens, in particular, in communication via the Internet. Different scenarios have been explored, inspired by typical cases encountered in clinical practice when taking in charge adolescents in crisis. Regarding the population of susceptible, suicidal and runaways, a model has been developed whose analysis shows that depending on the parameter values and the size of the initial population the system can evolve toward states where the populations of the suicidal and of the runaways are negligible or, on the contrary, reach appreciable values. Finally, we have shown that in absence of interactions between individuals the selfish to egalitarian transition in children occurs gradually, whereas in the presence of cooperativity the transition is more abrupt. Our results suggest that when taking in charge adolescents in a state of risk contagion and transition between mental states must be recognized as additional iatrogenic risks, which can be minimized by privileging small non-mixed groups. / Doctorat en Sciences médicales (Médecine) / info:eu-repo/semantics/nonPublished