Electrode-size dependent thresholds in subretinal neuroprosthetic stimulation

Retinal prostheses have shown promising results in restoring some visual perception to blind patients but successful identification of objects of different size remains a challenge. Here we investigated electrode-size specific stimulation thresholds and their variability for subretinal electrical stimulation. Our findings indicate the range of charge densities required to achieve identification of small objects and the object-size-specific scaling of stimulation threshold.

Using biphasic voltage-limited current stimuli provided by a light-sensitive microchip, we determined threshold charge densities for stimulation with variable electrode sizes. The stimulated activation of the retinal network was identified by recording the spiking of retinal ganglion cells in photoreceptor-degenerated mouse rd10 retinas. Stimulation thresholds were determined for cells with saturating stimulus response relationships (SRRs) but not for cells characterized by monotonically increasing or decreasing SRRs.

Stimulation thresholds estimated in rd10 retinas ranged between 100-900 µC cm^-2 for stimulation with small electrodes (30 µm diameter). Threshold charge density decreased with increasing electrode size and plateaued at 20 µC cm^-2 for an electrode diameter larger than 300 µm. This trend of decreasing threshold down to a plateau value was confirmed in wild-type mouse retina suggesting an underlying physiological source.

Our results suggest the following guidelines for retinal prosthetics employing biphasic current pulses. The encoding of small objects may be achieved through the activation of a confined set of different retinal ganglion cells, with individual stimulation thresholds spanning a wide range of charge densities. The encoding of increasing object sizes may be achieved by decreasing stimulation charge density.