Retinal and visual cortex distance from transcranial magnetic stimulation of the vertex affects phosphene perception

Recent studies claim that the perception of flashes of light (i.e., phosphenes) can be induced by stimulation of higher visual areas, including parietal cortex, suggesting a critical role of these regions in generating visual percepts. In this study, we show that transcranial magnetic stimulation (TMS) of even the vertex can induce phosphenes, but that their neural origins are likely to be a consequence of current spread into visual areas (e.g., retina or visual cortex). After vertex stimulation, subjects with smaller head circumferences—for whom the distances from the coil to retina and visual cortex are smaller—report a two-fold increase in perceiving phosphenes. In contrast, both smaller and larger headed individuals perceived phosphenes equivalently and on nearly all trials following TMS of early visual cortex. These results demonstrate a critical role of early visual areas but not higher ones in generating visual perceptions. These findings further suggest that phosphenes perceived from TMS of the vertex or parietal cortex arise from induced activity in the retina or nearby early visual cortex and warn against the use of the vertex as a control site for TMS experiments of visual perception.