Axonal stimulation affects the linear summation of single-point perception in three Argus II users

PURPOSE. Retinal implants use electrical stimulation to elicit flashes of light (“phosphenes”). Single-electrode phosphene shape has been shown to vary systematically with stimulus amplitude and frequency as well as the retinal location of the stimulating electrode, due to incidental activation of passing nerve fiber bundles. However, this knowledge has yet to be extended to paired-electrode stimulation. METHODS. We retrospectively analyzed 4402 phosphene drawings made by three blind subjects implanted with an Argus II Retinal Prosthesis. Phosphene shape (characterized by area, perimeter, major and minor axis length; normalized per subject) and number of perceived phosphenes were averaged across trials and correlated with the corresponding single-electrode parameters. In addition, the number of phosphenes was correlated with stimulus amplitude and neuroanatomical parameters: electrode-retina (“height”) and electrode-fovea distance (“eccentricity”) as well as the electrode-electrode distance to (“between-axon”) and along axon bundles (“along-axon”). Statistical analyses were conducted using linear regression and partial correlation analysis. RESULTS. Simple regression revealed that each paired-electrode shape descriptor could be predicted by the sum of the two corresponding single-electrode shape descriptors [Formula: see text]. Multiple regression revealed that pairedelectrode phosphene shape was primarily predicted by stimulus amplitude, electroderetina distance, and electrode-fovea distance [Formula: see text]. Interestingly, the number of elicited phosphenes increased with between-axon distance [Formula: see text] , but not with along-axon distance [Formula: see text]. CONCLUSIONS. The shape of phosphenes elicited by paired-electrode stimulation was well predicted by the shape of their corresponding single-electrode phosphenes, suggesting that two-point perception can be expressed as the linear summation of single-point perception. We also found that the number of perceived phosphenes increased with the between-axon distance of the two electrodes, providing further evidence in support of the axon map model for epiretinal stimulation. These findings contribute to the growing literature on phosphene perception and have important implications for the design of future retinal prostheses.