A computational model of electrical stimulation of the retinal ganglion cell

Localized retinal electrical stimulation in blind volunteers results in dis crete round visual percepts corresponding to the location of the stimulatin g electrode. The success of such an approach to provide useful vision depen ds on elucidating the neuronal target of surface electrical stimulation. To determine if electrodes preferentially stimulate ganglion cells directly b elow them or passing fibers from distant ganglion cells, we developed a com partmental model for electric field stimulation of the retinal ganglion cel l (RGC), In this model a RGC is stimulated by extracellular electrical fiel ds with active channels and realistic cell morphology derived directly from a neuronal tracing, Three membrane models were applied: a linear passive m odel, a Hodgkin-Huxley model with passive dendrites (HH), and a model compo sed of all active compartments (FCM) with five nonlinear ion channels, Idea lized monopolar point and disk stimulating electrodes were positioned above the cell. For the HH and FCM models, the position of lowest cathodal thres hold to propagate an action potential was over the soma, Brief (100 mu s) c athodic stimuli were 20% (HH with disk electrode) to 73% (FCM with point-so urce) more effective over the soma than over the axon. In the passive model , the axon is preferentially stimulated versus the soma, Although it may be possible to electrically stimulate RGC's near their cell body at lower thr esholds than at their axon, these differences are relatively small. Alterna tive explanations should be sought to explain the focal perceptions observe d in previously reported patient trials.