Phosphene

A phosphene is a visual percept produced without light entering the eye. People experiencing a phosphene may see flashes, spots, or geometric shapes in part or all of their visual field, even though there is no external light source. The phenomenon can occur under normal or unusual circumstances and is not tied to conscious visual processing from the environment.

Common triggers include mechanical stimulation of the retina or optic nerve (such as by pressing on or rubbing closed eyelids), electrical stimulation of retinal or cortical tissue, and noninvasive brain stimulation techniques such as transcranial magnetic stimulation. Phosphenes can also occur spontaneously in certain neurological or ophthalmic contexts, and they are sometimes reported during migraine with aura.

The underlying mechanisms vary by trigger. Mechanical stimulation is thought to depolarize retinal cells or alter neural activity in the retina, while electrical or magnetic stimulation directly activates neurons in the retina or visual cortex. In clinics and laboratories, phosphenes are used to map regions of the visual cortex and to calibrate or test retinal or cortical prostheses designed to restore vision in severe visual impairment.

Phosphenes are generally brief and benign, though repeated or intense stimulation may have risks depending on the method used. They should not be confused with afterimages or with visual phenomena caused by disease. The term derives from Greek words for light and showing.