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Michael P Barry, Gislin Dagnelie; Fluctuations in projected percept locations in subjects with retinal prostheses that use external cameras. Invest. Ophthalmol. Vis. Sci. 2015;56(7 ):749.
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© ARVO (1962-2015); The Authors (2016-present)
To characterize the nature of how the projected locations of percepts (PLPs) in subjects with retinal prostheses change over time.
Three end-stage RP patients implanted with the Argus® II epiretinal prosthesis participated in this study. These prostheses stimulate the retina based on a set 17° x 10° area selected within the camera’s 66° x 49° field of view. The center of this processed area is referred to as the camera position (CP). Participants touched a white square on a black background on a touchscreen monitor. Camera-screen distance was set before every trial run. Typical targets had sides spanning 5° of visual field. Locations of each square target and subject response, together with approximate camera locations, were used to calculate localization errors in degrees of visual field. Average errors were calculated for each trial run. CPs were adjusted to reduce errors when necessary. Trial runs with average errors of less than 1 degree were used to identify CPs that accurately reflected PLPs, relative to the camera. PLP estimates were confirmed by asking subjects to localize physical objects. Estimated PLPs were tracked over periods ranging 154-853 days. Significance of horizontal and vertical differences in PLPs for individual subjects across test session dates was evaluated using a bootstrap variation of ANOVA.
Test session date, and therefore time, had a significant effect on both horizontal and vertical components of PLPs for all three subjects (p < 10-4). PLP component spread ranged from 6.2°-24°, mean = 16°, standard deviation (SD) = 5.9°. Between-session rates of change for PLP components ranged 0°-1.6°/day, mean = 0.21°/day, SD = 0.36°/day. No measured PLPs displayed long-term stability.
PLPs displayed a lack of long-term stability in all three subjects. Periods of short-term stability were broken by shifts as dramatic as up to 1.6°/day. As prosthesis subjects with constant corrective feedback display much slower rates of adaptation to inaccurate CPs, and no adaptation without corrective feedback (ARVO 2014, #1817), regular recalibrations of prosthesis CPs are required to maintain subject hand-camera coordination.
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