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Katelyn Tsang, Javeria Hasan, Andrew Silva, Benjamin Thompson; The visual field location of phosphenes induced by transcranial magnetic stimulation (TMS) is precisely mapped to eye position. Journal of Vision 2020;20(11):410. doi: https://doi.org/10.1167/jov.20.11.410.
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© ARVO (1962-2015); The Authors (2016-present)
Cortical prostheses for vision restoration produce phosphenes by electrically stimulating groups of V1 neurons. The phosphenes are oculocentric; their perceived position is relative to the point of fixation. The aim of this study was to explore the fidelity of the mapping between the perceived location of electrically induced phosphenes and eye position. Phosphenes were induced using triple-pulse transcranial magnetic stimulation (TMS) of the primary visual cortex and eye position was monitored using an infrared gaze tracker. Nine participants were cued to fixate randomly selected locations within a 38.5º X 19.2º grid while keeping head position constant and indicated the perceived location of a TMS-induced phosphene using a computer mouse. A central fixation point was presented 8 times while all other fixation points (n = 35) were presented 3 times. The sequence of fixation locations was randomized. The TMS site was determined individually for each participant as the location eliciting a reliable central phosphene at the lowest stimulation intensity. The TMS stimulation site was held constant throughout the experiment using a BrainSight neuronavigation system. We observed a precise mapping between reported phosphene location and fixation location for all 35 grid locations in all nine participants (global mean distance of phosphene from fixation = 1.0º, SD across all fixation locations = 0.2º). No significant difference in phosphene location was found between the central and peripheral fixation locations, t(8) = 1.7, p = .13. Our results are consistent with prior studies involving visual prostheses and illustrate the oculocentric mapping of primary visual cortex. In addition, our results indicate that control of fixation is paramount in phosphene-related research.
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