Implantation of episcleral electrodes via anterior orbitotomy for stimulation of the retina with induced photoreceptor degeneration: An in vivo feasibility study on a conceptual visual prosthesis

Background. A visual prosthesis is a conceptual device designed to harnesses the function of residual afferent neurons in the visual pathway to produce artificial vision. Such implant, when applied to stimulate the vitreous surface of the retina, has proven feasible in producing the perception of light in both animals and humans. However the practicality of such device has been challenged by the difficulty of surgical access and the risks of damaging the neuroretina. Positioning a visual implant over the scleral surface of the eye could present a safer alternative but this stimulation modality has not been tested in diseased retinas and little is known about the altered electrophysiological properties of the retina in influencing the feasibility of such approach. Methods. Experimental photoreceptor degeneration was induced in four pigmented rabbit eyes with systematic administration of a retinotoxic agent, sodium iodate. A multielectrode array was implanted onto the surface of the sclera to target the central and peripheral parts of the retina via an anterior orbitotomy approach. The efficacy of retinal stimulation was assessed by recording electrical evoked potential over the primary visual cortex. Findings. The electrical evoked potentials were obtained from both injected and control eyes. The charge density thresholds were found to be similar in both groups and were below the bioelectric safety limit. Spatially differentiated cortical activation profiles were obtained from the central and peripheral retina and the pattern of activation corresponded to the retinotopography of the rabbit primary visual cortex. Conclusion. This study proves that episcleral stimulation of the retina is a feasible alternative to intraocular approaches for the development of a visual prosthesis for retinas with photoreceptor loss.