Connectivity patterns in visual processing: a TMS-based approach

Vision enables living organisms to perceive, interpret, and interact with the external world. Understanding the neural dynamics underlying the emergence of visual awareness represents one of the most challenging and compelling topics in cognitive neuroscience. In recent years, connectivity-based approaches have emerged as a powerful framework for probing cognitive functions and their neural underpinnings. Nevertheless, despite extensive investigation, the connectivity patterns supporting visual perception remain only partially understood. Against this background, the present doctoral thesis aimed to investigate the functional connectivity architecture of the visual system across three complementary levels of analysis, employing the combined methodology of Transcranial Magnetic Stimulation and electroencephalography (TMS-EEG). In the first study, we sought to advance the existing literature by characterizing communication patterns across specific areas during the emergence of visual awareness. Specifically, by stimulating the primary visual cortex to induce phosphenes, we investigated functional connectivity patterns associated with the presence and absence of perception following right- or left-occipital TMS. Phase synchronization between the stimulated site and proximal and distal cortical regions was quantified using the weighted Phase Lag Index (wPLI) across multiple frequency bands, revealing hemispheric asymmetries in connectivity patterns related to perceptual outcome. The second study shifted the focus to higher-order visual processing, specifically examining the controversial role of the dorsal visual pathway in object shape recognition. We employed cortico-cortical Paired Associative Stimulation (ccPAS) to selectively modulate connectivity within the dorsal pathway between the early visual cortex and the intraparietal sulcus. By measuring changes in behavioral performance on a task designed to dissociate global shape recognition from local feature processing, we tested whether strengthening dorsal pathway connectivity causally influences shape perception. This study was submitted as a Registered Report and received In-Principle Acceptance. However, due to the longer time requested, the results presented here represent a partial dataset relative to the complete registered protocol. In the third study, we extended our investigation to pathological conditions by examining how brain tumors affecting parieto-occipital regions impact visual system organization 3 and connectivity. Using TMS-EEG, we characterized network alterations associated with tumor growth as well as post-surgical reorganization. This approach allowed us to dissociate local alterations directly induced by the tumor from more distributed network level changes reflecting compensatory plasticity. Our data provide novel insights into how the brain undergoes plastic changes following structural pathological alterations, with potential implications for predicting surgical outcomes and informing rehabilitation strategies. Taken together, these three studies advance our understanding of the visual system's functional architecture across multiple dimensions: from the emergence of conscious visual perception to object recognition processes, and from healthy brain function to pathological reorganization. Beyond their theoretical relevance for models of visual cognition, these findings highlight the importance of adopting a network-based perspective when studying this complex and intriguing domain.