Understanding the origin of visual percepts elicited by electrical stimulation of the human retina

Background: The success of a retinal prosthesis for patients with outer ret inal degeneration (ORD) depends on the ability to electrically stimulate re tinal cells other than photoreceptors. Experiments were undertaken in human volunteers to ascertain whether electrical stimulation of cells other than photoreceptors will result in the perception of light. Methods: In two subjects, two areas of laser damage (argon green and krypto n red) were created in an eve scheduled for exenteration due to recurrent c ancer near the eye. In the operating room prior to exenteration, under loca l anesthesia, a hand-held stimulating device was inserted via the pars plan a and positioned over the damaged areas and normal retina. Subjects' psycho physical responses to electrical stimulation wore recorded. Results: In both subjects, electrical stimulation produced the following pe rceptions. Normal retina: dark oval (subject 1), dark half-moon (subject 2) ; krypton red laser-treated retina: small, white light (both subjects); arg on green laser-treated retina: thin thread (subject I), thin hook (subject 2). Histologic evaluation of the krypton red-treated retina showed damage c onfined to the outer retinal layers, while the argon green-treated area evi nced damage to both the outer and the inner nuclear layers. Conclusion: The perception produced by electrical stimulation was dependent on the retinal cells present. Electrical stimulation of the krypton red-ab lated area best simulated the electrically elicited visual perceptions of o ur blind, ORD patients, suggesting that the site of stimulation in blind pa tients is the inner retinal neurons.