Development of a silicon retinal implant: cortical evoked potentials following focal stimulation of the rabbit retina with light and electricity

There are currently many experimental efforts to develop a treatment for retinitis pigmentosa (RP) and age-related macular degeneration (AMD). The efforts reported here are part of the Retinal Implant Project, a collaborative effort of the Massachusetts Eye and Ear Infirmary and the Massachusetts Institute of Technology, to develop an independent functioning epiretinal prosthesis to restore vision in patients with disease of the outer retina such as RP and AMD. The epiretinal prosthesis will electrically stimulate the inner retina, bypass the damaged photoreceptors, and hopefully result in meaningful vision. One way to monitor the epiretinal implant is to record focal (stimulation of a small area of retina) evoked cortical potentials (EEP) upon electrical current stimulation. EEP recordings have 3 uses: (1) verify that focal retinal electrical stimulation sends a signal to the cortex that is similar to that elicited by light; (2) develop an animal model to chronically monitor the epiretinal implant; and (3) investigate optimal stimulus parameters. Five dutch pelted rabbits were placed under anesthesia and a 250 microm concentric bipolar stimulating electrode was introduced on the vitreal surface of the retina under visual guidance to record the EEP. Stimuli were charge-balanced pulses and recording electrodes were extradural. Focal VEPs were also recorded and compared to the focal EEP to ascertain cortical origin of the EEP, determine similarities of the EEP to the VEP and determine focality of electrical stimulus. EEP recordings were elicited in 3 out of the 5 rabbits. Current amplitudes which produced detectable responses ranged from 50-1000 microA. In our best series, amplitudes increased by 47-206 microV in increasing current from 50-600 microA. In comparing latencies of the focal EEP and focal VEP, the EEP latencies were 60-70 ms faster, favoring a cortical signal origin. In comparing amplitudes, VEP and EEP responses behaved similarly with changes in stimulus strength and stimulating frequency. In conclusion, EEPs were obtained with parameters of electrode size and current threshold level that would be used by the epiretinal implant, enabling use of the EEP response to monitor the epiretinal implant.