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 el ectrical stimulation in patients that are blind due to photoreceptor loss. To stimulate ganglion cells, epiretinally flat platinum microelectrodes emb edded in thin polyimide film were developed and tested in the cat. Method's : After removal of the lens and the vitreous body a thin microfilm electrod e 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 electrod e, which was pressed onto the retinal surface due to the tension of the cur ved polyimide film. In two eyes the tip of the electrode was fixed with cya noacrylate adhesive. The exterior part of the microelectrode film was direc ted under the skin towards the forehead which allowed fixation of the micro plug to a head fixation bolt. Retinal stimulation experiments were performe d within 1 week after implantation. Success of stimulation was assessed by recording neuronal activities from areas 17 and 18. Retinal microelectrodes were removed 7 weeks or longer after implantation. Results: Intraocular in flammation or retinal detachment were not observed after implantation of th e microelectrode film. In two eyes the tip of the microelectrodes dislocate d spontaneously within the first few days. The lowest threshold of electric al stimulation was 35 muA, corresponding to a charge transfer of 14 nC per phase. These values were ten times higher than those obtained by needle ele ctrodes used in prior experiments. Conclusions: Intraocular implanted fiat microelectrodes made of platinum and polyimide were well tolerated. Because of the flat configuration of the microelectrodes higher stimulation thresh olds than for needle electrodes were found, indicating insufficient contact to the retinal surface. An alternative shape and fixation technique is req uired res minimise electrodes' threshold of stimulation.