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Abstract

One of the key real-world challenges to our visual system is posed by cluttered scenes and occluded objects. To make sense of such scenes, local elements belonging to the same object need to be perceptually grouped, also referred to as spatial binding problem. However, it remains unknown how and where in the brain the local information is grouped together to give rise to a holistic percept. In the current study we addressed this question with a novel bistable motion stimulus developed by Anstis and Kim (2011) that consists of four pairs of dots coherently moving on a circular path. The stimulus causes perception to alternate spontaneously between two interpretations: local dot motion and global motion of two imaginary squares. Using functional magnetic resonance imaging (fMRI), we found that activity in the right parietal cortex correlated specifically with global as compared to local perception periods. To test for a causal role of parietal function in perceptual grouping, we used transcranial magnetic stimulation (TMS) to temporarily disrupt activity in two subregions of the parietal cortex. TMS over one of the subregions - the right anterior intraparietal sulcus (IPS) - specifically affected the global percept durations without affecting the local ones. Our results provide causal evidence that IPS may play a crucial role in perceptual grouping of local elements into a holistic percept, suggesting it to be a common anatomical locus of attention, perceptual grouping and perceptual selection processes.