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Jorg Sommerhalder, Angelica Perez Fornos, Farhad Hafezi, Marco Pelizzone, Argus II Study Group; On The Difficulty To Develop Appropriate Psychophysical Tests For The Evaluation Of Visual Prostheses. Invest. Ophthalmol. Vis. Sci. 2012;53(14):5551.
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To demonstrate and discuss the difficulty of finding pertinent psychophysical tests to measure benefits and visual improvements in blind patients wearing visual prostheses.
This study was carried out in 2 subjects implanted in our clinic with the Argus II® retinal prosthesis (a 60-electrode device by Second Sight Medical Products, Inc.; Sylmar, California). Subjects performed simple psychophysical tests: 2 on a computer screen (Grating Visual Acuity and EDTRS Visual Acuity) and 2 mobility tests (following a white line on a black carpet and walking towards a black door from 3m distance). Three conditions were tested: "blind" (no vision aid), using their retinal implant, and using a small handheld light detector (single photodiode with auditory feedback).
Subjects required extensive scanning to accomplish the tasks. In general, they achieved better performance with both devices than in the "blind" condition, except for the door task. Interestingly, subjects achieved better results with the single-channel light detector than with the Argus II device; nevertheless they clearly indicated a preference for their retinal prosthesis.
Several phenomena are likely to contribute to these observations. First, the tests are not adequate for demonstrating the advantages of a multichannel device (e.g., the tasks could be "easily" achieved with a single-channel device). Second, the two subjects that participated in this study are not among the best ("star-patients") in the trial, indicating that most probably only part of the information transmitted by their 60-channel device is grasped by their visual system. Third, the lack of form vision can be substantially compensated by scanning. Finally, retinal prostheses have limited image resolution and cover a limited visual field. Therefore, standardized testing methods, developed for normal and low vision patients, may not be appropriate to quantify visual performance with retinal prostheses.It is difficult to conceive appropriate psychophysical tests for the evaluation of visual prostheses, especially for measuring step-by-step improvements of devices in the development phase. Performance times are likely to be more important outcome variables than scores.
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