What Physiology Tells Us About Electrical Stimulation in Retinal Implants

Abstract

Purpose: : Polyimide chips with four platinum electrodes were designed as a prototype for subretinal implants in visual impaired patients affected by retinitis pigmentosa. Spacing and size of the electrodes were designed according to results of psychophysical experiments. The electrodes measured 50 µm in diameter and were equally spaced 150 µm apart. Their adequacy was evaluated using the chick retina as a model and intracellular monitoring of ganglion cell activity as a reporter of the quality of stimulation.

Methods: : Retina samples, separated from the pigment epithelium, were placed with photoreceptors in contact with stimulating electrodes and electrical stimulation consisted in charge balanced biphasic pulses. Ganglion cell activity was monitored using patch–clamp recording in current–clamp configuration.

Results: : Effects of stimuli were determined over a broad range of amplitude and duration. Rheobase and chronaxie were 7.2 ± 0.8 µA and 686 ± 33 µs respectively and corresponding charge densities were below 1 mC/cm². Blockade of the synaptic transmission using cadmium almost doubled the rheobase, suggesting that electrical stimulation triggered action potentials in ganglion cells through activation of proximal bipolar or amacrine cells. Shortening of the action potential latency observed in cadmium further supported this hypothesis.

Conclusions: : Action potentials were generated in retinal ganglion cells by stimulating from the photoreceptor layer with a multi–electrode chip. Current intensities and pulse durations allowing efficient ganglion cell stimulation were compatible with required physiological and electrical parameters. These results confirm the feasibility of ganglion cell stimulation using subretinal implants.

Keywords: electrophysiology: non-clinical • ganglion cells • retinal connections, networks, circuitry