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M. J. McMahon, A. Caspi, J. D. Dorn, K. H. McClure, M. S. Humayun, R. J. Greenberg; Spatial Vision in Blind Subjects Implanted With the Second Sight Retinal Prosthesis. Invest. Ophthalmol. Vis. Sci. 2007;48(13):4443.
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© ARVO (1962-2015); The Authors (2016-present)
Although electrical stimulation can produce localized spots of light (phosphenes) in patients blinded by retinal degeneration, some have questioned if stimulation of different retinal locations results in the perception of spatial patterns. The purpose of this work is to determine if electrical stimulation of individual electrodes in the Second Sight retinal prosthesis results in spatially localized percepts and to determine if the system can produce a perceptual image with resolution comparable to the spacing between electrodes.
Subjects were asked to place a pointer in the perceived location of phosphenes, and a tracking system was used to record their spatial position. We also performed a four alternative forced-choice orientation discrimination task that measured visual performance for square-wave gratings and examined the relationship between the number of active electrodes and visual performance.
The spatial position of phosphenes was consistent with the retinotopic location of the stimulating electrodes. The repeatability of the position judgments and the spatial resolution of the individual electrode positions was similar to the results obtained for sighted subjects who reported the location of analogous visual targets in the absence of visual feedback. When the prosthesis system was used to view patterned stimuli, orientation discrimination performance was greater than chance (i.e. 25% correct) for gratings finer than one grating cycle per array width, indicating the use of spatial information from individual electrodes. Results show that for spatial frequencies up to the limit determined by the electrode spacing, performance was significantly above chance. Further, we found a significant decrease in performance when only a subset of the electrodes in the array were stimulated.
Blind subjects can spatially resolve individual electrodes within the array of the implanted retinal prosthesis and can use the system to discriminate and identify oriented patterns. Moreover, visual performance improves with the number of stimulated electrodes. These results, obtained using a variety of techniques, strongly suggest that the development of retinal prosthesis systems with more electrodes will provide higher spatial resolution vision to blind subjects.
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