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Avinoam B Safran, Norman Sabbah, Nicolae Sanda, José-Alain Sahel; The blind man who saw his hands. Cross-modal plasticity revisited.. Invest. Ophthalmol. Vis. Sci. 2014;55(13):4147.
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© ARVO (1962-2015); The Authors (2016-present)
To describe a most remarkable, consistent cross-modal perceptual phenomenon experienced by a subject blinded by retinitis pigmentosa and fitted with a retinal prosthesis. This previously unreported phenomenon is informative on effects of cross-modal processes taking place in the visual cortex.
History taking in a 53 year old man with retinitis pigmentosa, who had for the last ten years his visual function limited to bare light projection. Four years ago, his right eye was fitted in our ophthalmology department with a retinal prosthesis (Argus® II, Second Sight Medical Products, Sylmar, California), which restored some visual function in the central 20 degree visual field, and improved visual acuity to 20/800.
The subject volunteered that, when standing in complete darkness, his prosthetic device being turned off, he consistently “saw” his hands and/or arms when he was moving them. The perceptual phenomenon was exclusively elicited by his limbs movements, and instantly disappeared when his limbs were still. The subject also noted that the perceived shape of his limbs was occluded in a small, well delineated area, approximately 20 degrees in diameter and precisely located in the direction he felt that he was “looking at”.
This observation deserves consideration, as it illustrates peculiar aspects of cross-modal processing in the visual cortex, including the impact own limbs movement on the generation of such synesthetic percepts. The central perceptually occluded area of these qualia presumably reflects either the peripheral predominance of non-visual sensory input in the retinotopic areas of the visual cortex, or the possibly prosthesis-induced reversal of the functional reorganization that had occurred in the deafferented visual cortex, an apparently unreported phenomenon.
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