Electrode Proximity and the Use of Opposing Stimuli on Electric Potential Resolution for Epiretinal Prosthesis Microelectrode Arrays

Abstract

Purpose: : Retinal prosthesis microelectrode arrays have been developed to replace the function of degenerated photoreceptor cells and directly stimulate ganglion cells. The spatial resolution created by microelectrode arrays are affected by electric potential spreading and close electrode proximity. Electric potential maps and gradients were used to investigate the spatial resolution of multiple stimulus patterns. New techniques using opposing stimuli improved resolution and increased electric potential gradients.

Methods: : A 16 electrode model for a 200+ electrode array was mounted in a model eye cell, immersed in Ames’ medium and maintained at 37 °C. Stimulation patterns were produced by electrodes using a cathodic first biphasic pulse at a previously determined maximum charge density of 100 µC/cm². Electric potential data were collected for 121 points across the array and plotted using the 3D plotting feature of Mathcad 14. Microsoft Excel was used to calculate gradients. Results were compared between experiments using similar cathodic stimulation patterns but with electrodes at varying proximities. Additional experiments used adjacent electrodes to create opposing stimuli by inducing various magnitudes of anodic first biphasic pulsation. Results were compared to experiments in which no opposing stimulus was present.

Results: : 3D electric potential maps of two point resolution experiments showed improved resolution with increased spacing between cathodic stimulated electrodes. Positive and negative gradients increased by 61% and 269%. Opposing stimuli of 26 and 39 µC/cm² applied to adjacent electrodes improved resolution of electric potential maps and increased gradients.

Conclusions: : Decreased electrode spacing diminishes electric potential resolution and gradients between stimulated electrodes. The use of opposing stimuli can increase gradients and improve electric potential resolution, ergo, is a prospect for improving shape resolution for blind patients implanted with epiretinal microelectrode arrays.

Keywords: retina • retinitis • retinal connections, networks, circuitry