Long-term Repeatability and Reproducibility of Phosphene Characteristics in Chronically Implanted Argus II Retinal Prosthesis Subjects.

PURPOSE:Previously published literatures of acute studies on few subjects have shown contradictory evidence on the reproducibility and characteristics of the elicited phosphenes, despite using the same stimulating parameters with epiretinal electrode arrays. In this study, we set out to investigate the long-term repeatilibity and reproducibility of phosphenes in subjects chronically implanted with the Argus II retinal prosthesis (Second Sight Medical Products, Inc., Sylmar, CA, USA). DESIGN:Retrospective interventional case series and reliability study. METHODS:Six Argus II subjects of >5 years implantation from a single site participated. The 4-electrode cluster ("quad") closest to fovea was stimulated in each subject with a fixed biphasic current. Perceived phosphenes were depicted relative to subjective visual field center. The stimulus was applied at reducing time intervals from 20 minutes to 1 second. Two sets of stimulations were performed on the same day and 2 further sets repeated on a separate visit >1 week apart. RESULTS:Each subject depicted phosphenes of consistent shapes and sizes, and reported seeing the same colors with the fixed stimulating parameters, irrespective of the interstimuli intervals. However, there is a wide intersubject variation in the phosphene characteristics. Four subjects drew phosphenes in the same visual field quadrant, as predicted by the quad-fovea location. Two subjects depicted phosphenes in the same hemifield as the expected locations. CONCLUSION:Phosphenes for each subject were consistently reproducible in all our chronically implanted subjects. This has important implications in the development of long-term pixelated prosthetic vision for future devices.