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Daw-An Wu, Junghyun Park, Shinsuke Shimojo; Stimulating “impossible” visual space with TMS. Journal of Vision 2006;6(6):382. doi: https://doi.org/10.1167/6.6.382.
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© ARVO (1962-2015); The Authors (2016-present)
When peering through binoculars, one is aware of the darkness that frames the field of view. However, no such darkness frames our everyday field of vision. When we move our eyes to gaze at the periphery, substantial portions of the retina fall under shadow. But rather than perceiving the dark insides of our skulls, we experience a shrinking field of awareness. Hayhoe & Williams (1984) and Cavanagh & Barton (VSS 2001) used afterimages and scleral illumination to show that retinal stimulation at these “impossible” locations in space is blocked from visual awareness.
Here, we directly stimulate visual cortex with transcranial magnetic stimulation, inducing percepts of brightness (phosphenes). Phosphenes are generally induced in the lower visual hemifield, in a retinotopically stable fashion. When participants look downward, these retinotopic locations can move outside the visual field. Participants were tested both in total darkness and under low illumination. They reported the spatial position and extent of their phosphenes with respect to various fixation points.
Almost all participants reported one or both of the following: 1) In the dark, phosphenes could extend into impossible locations. Participants reported phosphenes “inside my cheeks/chest”, “behind my ears”, etc. 2) Under room lights, these percepts were abolished. Instead, participants reported retinotopic shifts in location or the “squashing” of shapes to fit within the visual field.
TMS bypasses the mechanism which gates retinal input, allowing visual awareness to extend into ecologically impossible space. However, when environmental cues are present, it appears that spatial correspondences can be remapped to comply with visual field boundaries.
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