Quasi-monopolar electrical stimulation of the retina: a computational modelling study

Objective. In this study we investigated the feasibility of quasi-monopolar (QMP) electrical stimulation for retinal implant devices, using a computational model of the retinal ganglion cell layer. Approach. When used with hexagonally arrayed multiple electrodes, QMP stimulation is a hybrid of hexapolar and conventional monopolar stimulus modes. In hexapolar mode, each active electrode is surrounded by six guards which collectively return the stimulus current, whereas in monopolar mode the injected stimulus current is returned through a distant return electrode. The QMP paradigm, on the other hand, distributes the return current between the guard electrodes as well as the distant return. The electrodes tested were 25, 50 and 100 mu m in diameter, with hexagonally arranged centre-to-centre spacing of either double or quadruple this diameter. Main results. Simulation results indicated that electrode size had minimal effects on subretinal threshold currents, whilst electrode configuration and centre-to-centre spacing played major roles in determining thresholds and spatial activation patterns. Threshold charge densities for 50 and 100 mu m electrodes were generally within the safe limit. Significance. We found that QMP stimulation offers greater advantages compared to monopolar and hexapolar stimulation, in that it combines the low thresholds of monopolar stimulation with the localized spatial activation achieved with hexapolar electrodes during parallel stimulation.