Insights into the Interconnected Brain during (Multi-)sensory Processing - A concurrent TMS-fMRI Approach

The ability to appropriately respond to sensory information from our surroundings relies on the dynamic interplay between different and distributed brain regions, which is flexibly adapted according to current contexts and demands. By allowing direct monitoring of local and distal effects of transcranial magnetic stimulation (TMS), the concurrent application of TMS and functional magnetic resonance imaging (fMRI) provides a causal interventional approach to investigate this interconnected nature of the brain. In this dissertation, we used this methodology to investigate the neural mechanisms underlying (audio-)visual processing under different cognitive and experimental settings. In particular, given the functional heterogeneity of the intraparietal sulcus (IPS) in a number of different cognitive functions, a special focus was given to the functional role of this region during such processes. As a first step, we evaluated the causal involvement of the IPS during crossmodal deactivations by applying continuous repetitive TMS at different intensity levels to the right IPS and at the Vertex during three different sensory contexts (visual, auditory and fixation). Second, by engaging the attentional network in a demanding visual detection task we investigated how TMS at the right IPS influenced task-related activations, by applying bursts of TMS pulses over the right IPS and during a Sham condition. Moreover, given that additional sensory information might influence task performance in a beneficial or in a detrimental way, we further manipulated the bottom-up sensory context by introducing two different auditory contexts (present vs. absent). Third, to evaluate the differential effects of stimulating low-level sensory areas and higher-order association cortices, we compared the consequences of parietal and occipital stimulation on task-related activations during an identical experimental setting. Lastly, keeping the same visual detection task, we assessed the role of the IPS during perceptual decisions by categorizing participants’ responses into hits, misses, false alarms and correct rejections and comparing conditions with matched visual input but different behavioural response categories and vice versa. Overall, our results provide causal evidence for the involvement of the right IPS in different stages of sensory processing. Moreover, they also reflect the ability of the concurrent TMS-fMRI approach to divide a global task-related network into those elements that are specifically associated to the targeted area