Abstract
Introduction: Retinal implants provide artificial vision to blind individuals through electrically stimulating remaining non-photoreceptor retinal cells. For epiretinal implants, placed over the ganglion cell layer, individual electrodes produce elongated ‘streaks’ due to the unselective stimulation of underlying ganglion axons (Beyeler, 2019). Here, to examine whether these axonal streaks explain the poor spatial acuity of prosthetic patients, we measured two-point discrimination performance in three patients implanted with an Argus 2 epiretinal implant (Second Sight Medical Products Inc). Methods: On each trial two electrodes were simultaneously stimulated (0.45 um pulse width, 6-20 Hz pulse train, 250-500ms duration, current amplitude 2x threshold). Participants verbally reported the number of distinct percepts they saw. Results: A regression analysis found that current amplitude, physical distance, distance along the axon, and distance between axons all played a significant role in determining whether participants saw one or two percepts. Conclusions: Participants were less likely to see two distinct percepts when electrodes were physically close or lay close to the same axon bundle. Electrodes with high stimulation thresholds were also less likely to produce distinct percepts. Thus electrode pairs can merge into a single percept when (1) current fields overlap, (2) their current fields stimulate the same axonal bundle, or (3) the elongated percepts overlap.