Axon-bundle activation and phosphene streaks

A dominant distortion in epiretinal stimulation: an electrode over the nerve-fiber layer excites not only the ganglion-cell body beneath it but the axons of distant cells passing by, whose somata lie elsewhere in the retina. Because those axons follow arcuate nerve-fiber-bundle trajectories toward the optic disc, the percept is an elongated arc or comet streak rather than a focal dot.

The effect is predictable from retinal nerve-fiber anatomy and largely explains why epiretinal image quality is poor. Mitigations include very short (submillisecond) pulses that preferentially activate cell bodies over axons, and encoding models that account for the streak so the intended shape survives.