Resolution of the Epiretinal Prosthesis is not Limited by Electrode Size

Epiretinal prostheses for the blind bypass diseased photosensitive cells in the retina, directly stimulating retinal neurons electrically and evoking signals that are relayed to the brain. Current clinical implants have few electrodes and provide limited visual acuity. Acuity may be improved by identifying electrode array design features and operational details that enhance or interfere with visual percept formation. We labeled all retinal ganglion cells in whole mount retina with a calcium reporter and then measured the number and pattern of cells responding, over a range of electrode diameters and stimulus durations. Span of the response scaled with electrode diameter for electrodes 60 mu m and larger. Short stimulation pulse widths selectively activated cells nearest the electrode. Our measurements in the salamander retina suggest that the spatial resolution is 150 mu m, which on a human retina is equivalent to 0.55 degrees of human visual field and corresponding Snellen acuity of 20/660. Reading large print could be possible with such a prosthesis.