Evoked cortical potentials after electrical stimulation of the inner retina in rabbits

Abstract

 Background: Electrical stimulation of the retina using implantable devices may be one possible approach in the treatment of blindness causing progressive degeneration of the outer retina. Stimulation of ganglion cells or fibers could be achieved by epiretinal positioning of a microelectrode array as one component of a retinal prosthesis. Experiments were performed in rabbits to determine whether it is possible to elicit cortical responses with current pulses delivered via an epiretinal placed microelectrode array. Methods: Polyimide-based microelectrode arrays with platinum electrodes and silicon-based TiN electrode arrays were implanted and placed onto the retinal surface of healthy pigmented rabbits after vitrectomy. The devices were temporarily fixated under PFCL and connected to constant current sources delivering bi- or monophasic current pulses. Cortical evoked potentials were recorded using subcutaneously implanted EEG electrodes over the visual cortex. Results: The surgical procedures for implantation and fixation could be performed without serious complications. The electrode array could be placed in the area of the visual streak. Cortical potentials could be recorded after pulse train stimulation either with biphasic or with monophasic pulses. At threshold, pulses of 10 µA/phase and 100 µs/phase were necessary for detection of an electrically evoked cortical potential (EEP). Charge delivery at threshold varied between 0.1 and 0.3 nC/phase and charge densities varied between 1 and 12 µC/cm². The EEP amplitude increased with increasing stimulus strength. The cortical response could be completely blocked with a retrobulbar injection of 4 ml lidocaine 2%. Conclusion: Constant current pulses delivered via Retina Stimulator devices equipped either with platinum electrodes or with TiN electrodes placed onto the inner retinal surface were able to elicit cortical potentials in the rabbit visual system. At threshold the charge delivery seems to be in a safe range.