Phosphene

The elementary percept of a visual prosthesis: a spot of light evoked by electrical (or optogenetic) stimulation in the absence of light. Its retinal or cortical origin fixes its location in the visual field; brightness grows with stimulus charge and frequency, and apparent size grows with current, so a phosphene is a controllable but crude pixel.

Real phosphenes depart badly from ideal dots. They vary in shape, color, and brightness across electrodes, may appear as arcs or streaks, fade under sustained drive, and interact when neighbors are co-activated. Turning an electrode grid into a legible image is therefore not simple pixel display but a perceptual-engineering problem.