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Functional abnormalities of the default network in autism


One of the most striking and debilitating features of autism is the profound impairment in social and emotional functioning. In recent years, the emergence of modern cognitive neuroscience techniques has led to a greater understanding of the neural bases of such abilities in healthy control subjects. However, very little is known regarding the neural bases of the impaired social and emotional functioning in individuals with autism. In the present series of studies, the functioning of the default network was examined across various task and non-task conditions. This network is comprised of the medial prefrontal cortex (MPFC), retrosplenial cortex/posterior cingulate cortex (RSC/PCC), precuneus (PrC), and angular gyrus (ANG), among other regions, and typically activates during performance of tasks of a social, emotional, or self-referential introspective nature (e.g., social perception, emotion processing, mentalizing tasks, autobiographical recall, etc.). Furthermore, the default network maintains high levels of metabolic activity at rest, in the absence of any task demands, suggesting that ongoing unconstrained thoughts of a social, emotional, and introspective nature might be the natural, default state of the mind. In Chapter II, the default network activity in autism was examined during a resting condition and an implicit emotion processing condition. In both of these conditions, reduced levels of activity were found in regions of the default network. Furthermore, the degree of resting functional abnormality in the MPFC correlated to a clinical measure of social impairment. In Chapter III, these findings were extended by demonstrating reduced levels of functional connectivity within the default network at rest, with the reduction being particularly pronounced in the MPFC and left ANG. These reductions were not a pervasive feature of the resting brain, as no abnormality was found in a different network (i.e., the dorsal attention network). In Chapter IV, in order to determine whether default network regions are capable of being engaged under particular experimental circumstances, activity was measured during an explicitly-defined and experimentally-constrained self- and other-reflection task. Although subtle task-specific abnormalities in autism were found, the overall level of functional activity across all conditions was remarkably similar between groups. All together, these findings suggest that while the default network often does not engage under typical circumstances in individuals with autism, it is in fact capable of responding, given particular highly- specified experimental constraints. Further investigation of the nature of this engagement deficit might prove instrumental in elucidating the causes of autism, and may help to refine behavioral treatments

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