Electric field stimulation of bipolar cells in a degenerated retina--a theoretical study

Subretinal implants are the subject of clinical investigation for their ability to evoke useful visual sensations in blind individuals via electrical stimulation of the diseased retina. We investigated the spatial characteristic of the retinal polarization obtained by electric field stimulation through a subretinally located monopolar electrode array and bipolar electrode array. By combining electric potential simulation through a boundary element method with a segmented cell model, we computed the membrane voltage at the axon terminal of the bipolar cells as a function of the axon length (50-110 microm) and the electrode diameter. We found that short OFF bipolar cells are predominantly addressed by small bipolar electrodes (diameter between 60 and 100 microm) and by using a short duration ( < 150 micros) of the stimulating voltage pulse. Long ON cells are best addressed by large monopolar electrodes (diameter > 100 microm) and a long pulse duration ( > 150 micros). However, the low selectivity of the electric field stimulation with regard to the cell length does not enable the individual depolarization of long OFF cells and short ON cells. When the stimulation must take place at multiple retinal sites simultaneously, the bipolar electrode arrays allow for higher spatial modulation of the polarization of the axon terminal than the monopolar arrays.