Determinants of spatial resolution in retinal prostheses: understanding the gap between geometry and vision

Retinal prosthetic systems have emerged as a promising technology for restoring partial vision in patients with severe photoreceptor degeneration. Over the past two decades, development in epiretinal, subretinal, and suprachoroidal devices has progressed substantially, with several representative platforms demonstrating the feasibility of restoring light perception, object localization, motion discrimination, and limited-form vision in clinical settings. Despite these advances, the visual acuity achieved by existing retinal prostheses remains markedly lower than predictions based on device geometry alone. Particularly, a persistent gap exists between the theoretical spatial resolution implied by electrode or pixel spacing and clinically reported visual performance. This review examines the major factors limiting effective spatial resolution in retinal prosthetic systems, with particular emphasis on the resolution gap. Collectively, available evidence indicates that while electrode spacing may be considered a critical determinant of theoretical resolution, it does not represent a direct surrogate for clinical visual acuity. Consequently, improving prosthetic vision requires denser arrays alongside selective neural recruitment, more stable electrode-retina coupling, and stimulation strategies that optimize functionally meaningful percepts. Elucidating the resolution gap and its underlying determinants will be essential for developing next-generation retinal prosthetic systems that provide more effective visual restoration.