gms | German Medical Science

Purpose: To demonstrate that magnification and image processing in retinal prosthesis with an external camera and processor can enable a visual acuity well beyond the limit set by theoretical resolution of the implanted array.

Methods: To date, more than 60 patients blinded by outer retinal dystrophies have received an Argus epi-retinal prosthesis (Second Sight, Sylmar, CA). The Argus II System, which is commercially available in the European Economic Area and in the Middle East, is the only retinal implant approved for commercial use in these patients anywhere in the world. In a retinal prosthesis, the retina is stimulated based on the light level in the receptive field of the electrode, directly or (as in the case of the Argus II) via a video camera. As a result, the spatial resolution is nominally set by the number of electrodes and the distance between neighbouring electrodes. To date the best nominal acuity achieved has been 20/1260. In the current study, the video image acquired by a high resolution camera was processed before being wirelessly transmitted to the implant. While the field-of-view of the cells in the retina covered by the Argus II array is about 20 degrees diagonally, in the current experiment the subject was able to reduce or magnify the image in a range from 0.4x to 16x using a remote hand-held controller (Logitech R-400). In addition, image enhancement was done by extracting global features in the image. Visual acuity was measured using a grating visual acuity test in which square-wave gratings at four orientations were presented on a computer monitor and subjects were required to report the orientation of the bars within the 5 second time limit. In addition, the subject was asked to read short words from a notebook with 2.3 cm letters.

Results: Using a high magnification of 16x the subject’s visual acuity was measured at 1.0 logMAR (20/200).

For the reading task, the subject chose to use 4x magnification and was able to read accurately from a notebook at a distance of 30 cm.

Conclusions: Variable magnification and other image processing strategies can extend the functionality of a retinal prosthesis beyond the spatial resolution set by the number and spacing of the electrodes. This may prove to be a unique advantage of a retinal prosthesis that employs an external camera and processor, such as the Argus II.