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Saddek Mohand-Said, Avi Caspi, Francesco Merlini, Brian Coley, Jessy Dorn, Gregoire Cosendai, Robert J. Greenberg, Argus II Study Group; Comparison Of Etdrs, Landolt C, And Grating Visual Acuity Tests Between Sighted Volunteers Using A Pixelized Image Simulator And Blind Subjects Implanted With The ArgusTm Ii Retinal Prosthesis. Invest. Ophthalmol. Vis. Sci. 2011;52(14):4931.
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Visual acuity is a quantitative objective method to assess ophthalmology procedures. The purpose of this study is to compare acuity tests with different stimuli to evaluate the non-natural vision provided by a retinal prosthesis implant.
Visual acuity was measured using the following tests: (1) Grating visual acuity: square-wave gratings at four orientations were presented on a computer monitor and subjects were required to report the orientation of the bars. (2) Letter visual acuity: letters from the standard ETDRS Chart were presented one at a time. Five letters from each size were randomly presented and the subject was asked to identify the letter. (3) Landolt C optotype: a four alternative forced-choice test, the subject is required to report the location of the gap in the optotype.Three sighted volunteers performed the same tests with a virtual reality simulator with an instantaneous field-of-view of 1.8°, which matched the 525 µm distance between adjacent electrodes of the Argus II implant, yielding a theoretical visual acuity of 2.03 logMar according to the Nyquist sampling criteria.
Using a pixelized image simulator, observers achieved acuity of 2.1 logMar on the Letter test, which is close to the Nyquist limit. On the Grating and Landolt C tests they were able to score between 1.6 and 1.9 logMar, which is better than the sampling limit. Two blind subjects implanted with the Argus II scored 2.1 and 2.5 logMar, respectively, on the Letter test. On the Grating test the same subjects scored 1.8 and 2.3 logMar, while with the Landolt C test they scored 2.1 and 2.5 logMar.
The ETDRS-based Letter test performed on blind subjects using a retinal prosthesis and sighted subjects using a prosthesis simulator yielded acuity measures up to the resolution of the device. However, the results on the Landolt C and Grating tests indicate that it is possible, using a closed set of stimuli, to beat the sampling limit by integrating visual information and information from head scanning.
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