Sequential epiretinal stimulation improves discrimination in simple shape discrimination tasks only

Objective. Electrical stimulation of the retina can elicit flashes of light called phosphenes, which can be used to restore rudimentary vision for people with blindness. Functional sight requires stimulation of multiple electrodes to create patterned vision, but phosphenes tend to merge together in an uninterpretable way. Sequentially stimulating electrodes in human visual cortex has recently demonstrated that shapes could be 'drawn' with better perceptual resolution relative to simultaneous stimulation. The goal of this study was to evaluate if sequential stimulation would also form clearer shapes when the retina is the neural target.Approach. Two human participants with retinitis pigmentosa who had Argus^®II epiretinal prostheses participated in this study. We evaluated different temporal parameters for sequential stimulation and performed phosphene shape mapping and forced choice discrimination tasks. For the discrimination tasks, performance was compared between stimulating electrodes simultaneously versus sequentially.Main results. Phosphenes elicited by different electrodes were reported as vastly different shapes. For sequential stimulation, the optimal pulse train duration was 200 ms when stimulating at 20 Hz and the optimal gap interval was tied between 0 and 50 ms. Sequential electrode stimulation outperformed simultaneous stimulation in simple discrimination tasks, in which shapes were created by stimulating 3-4 electrodes, but not in more complex discrimination tasks involving ≥5 electrodes. The efficacy of sequential stimulation depended strongly on selecting electrodes that elicited phosphenes with similar shapes and sizes.Significance. An epiretinal prosthesis can produce coherent simple shapes with a sequential stimulation paradigm, which can be used as rudimentary visual feedback. However, success in creating more complex shapes, such as letters of the alphabet, is still limited. Sequential stimulation may be most beneficial for epiretinal prostheses in simple tasks, such as basic navigation, rather than complex tasks such as novel object identification.