PUB - Publikationen an der Universität Bielefeld

To interact successfully with our environment, our brain must process many incoming sensory stimuli in parallel, yet we have only one unified percept. The process of combining these multiple stimuli into a single percept is called multisensory integration (MI). MI acts as a stimulus enhancer because the detection of parallel stimuli belonging to the same object is faster and more accurate than when only one stream of stimuli is presented. The faster detection of multimodal stimuli is neurally organized in heteromodal convergence zones, i.e., brain regions that can process more than just one stimulus modality and assemble multimodal information. These convergence zones are located in the posterior parietal cortex, lateral intraparietal, and superior temporal sulcus. Depending on the task demands and the complexity of the MI-stimuli, the attentional resources that are allocated are varying. While, salient, relative simply multimodal stimuli are integrated rather early - so-called bottom-up processing, more complex stimuli are integrated rather late in a top-down processing. Bottom-up integration is almost automatic and involves primary sensory areas and superior colliculi, whereas top-down integration involves higher-order association areas and frontal areas for cortical feedback projections. Attention-deficit/hyperactivity disorder (ADHD) is a neurodevelopmental disorder with age-inappropriate symptoms of inattention and/or hyperactivity. ADHD has long been considered a childhood-disorder, but current evidence shows that 40-50% of patients continue to have symptoms into adulthood resulting in a global prevalence of 2.8%. Neurobiologically, ADHD is currently considered as a network disorder with alterations in the task-negative default mode network, the salience network, and the attentional control network. Despite the core symptoms, ADHD is characterized by sensory processing alterations in the sensory modalities, especially in the auditory and visual modalities towards a higher sensitivity, resulting in increased auditory and visual distractibility. Since MI is dependent on the attentional allocation, and ADHD is a disorder of inattention and alterations in unimodal sensory processing, the studies presented in this thesis follows the question of potential consequences for multimodal processing by investigating bottom-up and top-down MI separately in adult ADHD. In addition, MI is associated with functional and structural connectivity to examine a potential overlap between ADHD pathophysiology and MI-abilities. The first study examines whether bottom-up and/or top-down MI is deficient in ADHD, using the sound-induced flash illusion for the former and the McGurk illusion for the latter. Additionally, resting-state functional connectivity was associated with the McGurk performance. As a result, patients with ADHD showed intact MI for bottom-up integration and significantly less MI for top-down integration compared to healthy controls. General functional connectivity revealed an overactive connectivity in DMN-regions and between sensory regions in patients with ADHD but not in healthy controls. Top-down MI across the whole sample was inversely correlated in functionally connected heteromodal, frontal, and temporal regions. The second study examined overall whole-brain tractoraphic structural network integrity and correlated structural connectivity with McGurk-performance. Compared to controls, patients with ADHD elicited higher connectivity in temporal and occipital networks and lower connectivity in fronto-temporal, fronto-limbic and fronto-insular networks. McGurk performance was associated with networks typically recruited for bottom-up integration, while the healthy control group showed expected connectivity patterns for top-down MI. The third study addressed bottom-up integration, but with an additional task demand, namely a perceptual load visual search task. As a result, MI was similar in ADHD patients compared to healthy controls. However, application of the race model revealed a difference in sensory accumulation at high perceptual load in ADHD. Taken together, the results of the present thesis show that bottom-up MI for simple stimuli is intact in adult patients with ADHD, although the sensory accumulation is altered compared to healthy controls. For complex stimuli, where top-down integration is required, ADHD is marked by deficient MI. Here, the dominance of the auditory modality plays a central role as most often rely on the auditory sense in bimodal scenarios. Neurobiologically, there is an overlap in the pathophysiology of ADHD and the regions responsible for MI. Overactive DMN activity may block the natural advantage for multimodal stimulus processing. Furthermore, the results are discussed in the context of perceptual decision making, dopaminergic sensory processing and developmental aspects of ADHD. Overall, the present thesis advances our understanding of multisensory behavior in adult ADHD and the underlying functional and structural networks.