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Kelly Webster, Tony Ro; Phosphene perception from transcranial magnetic stimulation (TMS) over the vertex. Journal of Vision 2016;16(12):1142. doi: 10.1167/16.12.1142.
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© 2017 Association for Research in Vision and Ophthalmology.
TMS, a technique that disrupts neural processing and can induce the perception of flashes of light (i.e., phosphenes) by stimulation of visual areas, is a valuable tool for studying the mechanisms underlying human visual perception. Recent studies claim that phosphenes can be induced even by parietal stimulation (Marzi et al., 2009; Fried et al., 2011; Mazzi et al., 2014; Bagattini, Mazzi, & Savazzi, 2015), raising concerns about the many studies that use the parietal lobe or vertex as control sites to rule out non-specific effects of the TMS. In this study, we assessed whether phosphenes induced by TMS of the vertex are likely to be a consequence of current spread into the retina or visual cortex. TMS was applied over V1 or the vertex at intensities that reliably produced phosphenes over V1 and found that subjects with smaller head circumferences reported a nearly threefold increase in perceiving phosphenes from vertex stimulation compared to subjects with larger head circumferences. In contrast, there was no significant difference between these two groups in the percentage of phosphenes reported from V1 stimulation. Because distinct phosphenes cannot be reliably induced from TMS of the retina, these results strongly suggest that phosphenes perceived from TMS of the vertex or parietal cortex arise from current spread into nearby visual cortex and indicate that the use of the vertex as a control site for TMS experiments of visual perception may be highly problematic.
Meeting abstract presented at VSS 2016
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