Retinal implants give blind people artificial vision by electrically activating non-photoreceptor retinal cells. Individual electrodes in epiretinal implants placed above the ganglion cell layer create elongated ‘streaks’ due to the unselective activation of underlying ganglion axons (Beyeler, 2019). For a study, researchers examined two-point discrimination performance in 3 patients implanted with an Argus 2 epiretinal implant to see if these axonal streaks explain the low spatial acuity of prosthetic patients (Second Sight Medical Products Inc). In each trial, two electrodes were activated simultaneously (0.45 um pulse width, 6-20 Hz pulse train, 250-500ms duration, current amplitude 2x threshold). The number of unique percepts seen was reported verbally by participants. According to the regression analysis results, current amplitude, physical distance, distance along the axon, and distance between axons all had a significant influence in deciding whether participants observed one or two percepts. Participants were less likely to detect two different percepts when electrodes were physically close or laid adjacent to the same axon bundle. High stimulation threshold electrodes were also less likely to create discrete percepts. When current fields overlap, their current fields excite the same axonal bundle or the elongated precepts overlap; electrode pairs can merge into a single percept.
Source: jov.arvojournals.org/article.aspx?articleid=2778470
