Feasibility of extraocular stimulation for a retinal prosthesis.

Background: We present a new approach to developing a retinal prosthesis for blind patients based on extraocular stimulation of the eye with disc electrodes.

Methods: Experiments to assess the feasibility of using extraocular stimulation in a retinal prosthesis were carried out in anaesthetised adult cats (n=6). A craniotomy and lateral orbital dissection were performed. Ball or disc electrodes were placed on the posterior scleral surface of the eye after incision of the periorbita. Cortical potentials evoked by electrical stimulation with these electrodes were recorded at the primary visual cortex. The viability of adapting the Nucleus 24 auditory brainstem implant (ABI) as an extraocular retinal prosthesis was also investigated.

Results: Electrodes placed on the exterior of the eye could reliably evoke visual cortex responses for a variety of configurations. Threshold currents for eliciting an evoked response were lower than 100 microA with single pulses. Strength-duration curves and cortical activation maps were obtained for different stimulus paradigms. It was possible to excite the retina to evoke a cortical response using the electrodes and stimulus capabilities in a standard Nucleus 24 ABI.

Conclusions: It is possible to electrically stimulate the retina with electrodes placed in an extraocular location. Threshold currents required to elicit a response were low, and comparable to epiretinal implants. Prototype electrodes, and a potential implant, were found to be effective at retinal stimulation.