Egocentric processing in the roll plane and dorsal parietal cortex: A TMS-ERP study of the subjective visual vertical.

The intraparietal sulcus within the dorsal right posterior parietal cortex is associated with spatial orientation and attention in relation to egocentric reference frames, such as left or right hemifield. It remains unclear whether it plays a causal role in the human in the roll plane (i.e. when visual stimuli are tilted clockwise or anticlockwise) which this is an important aspect of egocentric visual processing with clinical relevance in vestibular disorders. The subjective visual vertical (SVV) task measures the deviation between an individual's subjective vertical perception and the veridical vertical, involves the integration of visual, and vestibular information, and relies on a distributed network of multisensory regions that shows right lateralization and inter-areal inhibition. This study used combined TMS-EEG to investigate the role of the human dorsal parietal cortex in verticality perception using the SVV task in darkness. Participants were sorted according to their baseline bias at this task i.e. those with either a slight counterclockwise versus clockwise bias when judging a line to be truly vertical. Right parietal TMS facilitated verticality perception, reducing the difference between groups. ERPs suggested that the behavioral TMS effect occurred through normalizing individual SVV biases, evident frontally and late in the trial, and which was abolished after right parietal TMS. Effects were site and task specific, shown with a homologous left hemisphere control, and a landmark task performed on the same stimuli. These results support a right lateralization of visual-vestibular cognition and a distinct representation of the roll plane for egocentric processing in dorsal parietal cortex.