Two-dimensional and three-dimensional modeling of retinal prosthesis stimulation of bipolar cells

Purpose. Focal electrical stimulation of the retina will benefit from optimal design of electrodes and stimulus parameters; such optimization can be estimated using computational models. This study extends previous work in two dimensions, and utilizes a 3-D compartmental model of a bipolar cell (BC) subjected to electric fields generated by finite monopolar and bipolar electrodes. Methods. Based on confocal microscope images of a' cone BC (provided by Dr. S. DeVries, NWU), a 3-D model was constructed in the NEURON simulation environment; biophysical properties were assigned based on major currents reported in the literature. Extracellular field potentials associated with two microelectrode geometries were simulated using a finite-element model (ANSYS) of a subretinal implant in a human eye. Electric field potentials were used to calculate transmembrane currents throughout the model BC, and membrane potential (Vm) was evaluated at a point near the axon terminals. Results. The 3-D results are not strongly dependent on the orientation of the dendritic arbor, even for the inhomogeneous electric field associated with bipolar electrodes. Minimum charge transfer, good for electrode lifespan and minimum power requirements, can be achieved using asymmetric stimuli with a relatively high-amplitude, short duration initial anodic phase followed by a low-amplitude charge-balancing cathodic phase.