Implantation of retina stimulation electrodes and recording of electrical stimulation responses in the visual cortex of the cat

Background: Simple basic visual perception may be restored by epiretinal electrical stimulation in patients that are blind due to photoreceptor loss. To stimulate ganglion cells, epiretinally flat platinum microelectrodes embedded in thin polyimide film were developed and tested in the cat. Methods: After remo-val of the lens and the vitreous body a thin microfilm electrode array was implanted through a corneoscleral incision in the cat eye (n=4). In two eyes no further attempt was made to fixate the tip of the electrode, which was pressed onto the retinal surface due to the tension of the curved polyimide film. In two eyes the tip of the electrode was fixed with cyanoacrylate adhesive. The exterior part of the microelectrode film was directed under the skin towards the forehead which allowed fixation of the microplug to a head fixation bolt. Retinal stimulation experiments were performed within 1 week after implantation. Success of stimulation was assessed by recording neuronal activities from areas 17 and 18. Retinal microelectrodes were removed 2 weeks or longer after implantation. Results: Intraocular inflammation or retinal detachment were not observed after implantation of the microelectrode film. In two eyes the tip of the microelectrodes dislocated spontaneously within the first few days. The lowest threshold of electrical stimulation was 35 µA, corresponding to a charge transfer of 14 nC per phase. These values were ten times higher than those obtained by needle electrodes used in prior experiments. Conclusions: Intraocular implanted flat microelectrodes made of platinum and polyimide were well tolerated. Because of the flat configuration of the microelectrodes higher stimulation thresholds than for needle electrodes were found, indicating insufficient con-tact to the retinal surface. An alter-native shape and fixation technique is required to minimise electrodes’ threshold of stimulation.