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Purpose: The goal was to evaluate how perceptual thresholds are related to electrode impedance, electrode size, the distance of electrodes from the retinal surface, and retinal thickness in six subjects blind as a result of retinitis pigmentosa, who received epiretinal prostheses implanted monocularly as part of a U.S. Food and Drug Administration (FDA)-approved clinical trial., Methods: The implant consisted of an extraocular unit containing electronics for wireless data, power recovery, and generation of stimulus current, and an intraocular unit containing 16 platinum stimulating electrodes (260- or 520-microm diameter) arranged in a 4 x 4 pattern. The electrode array was held onto the retina by a small tack. Stimulation was controlled by a computer-based external system that allowed independent control over each electrode. Perceptual thresholds (the current necessary to see a percept on 79% of trials) and impedance were measured for each electrode on a biweekly basis. The distance of electrodes from the retinal surface and retinal thickness were measured by optical coherence tomography on a less regular basis., Results: Stimulation thresholds for detecting phosphenes correlated with the distance of the electrodes from the retinal surface, but not with electrode size, electrode impedance, or retinal thickness., Conclusions: Maintaining close proximity between the electrode array and the retinal surface is critical in developing a successful retinal implant. With the development of chronic electrode arrays that are stable and flush on the retinal surface, it is likely that the influence of other factors such as electrode size, retinal degeneration, and subject age will become more apparent. (ClinicalTrials.gov number, NCT00279500.).