Phosphene

a visual sensation that occurs in man without the action of light on the eyes.

Phosphenes may appear spontaneously in the dark, or they may be induced by mechanical pressure on closed eyes, by the chemical action of psychotropic agents on the central nervous system, or by the electrical excitation of the retina through electrodes placed on the temples. They can also be induced by direct electrical excitation of the visual centers of the cerebral cortex.

Phosphenes may be bluish, greenish, yellowish, or orange in color; their shapes are varied. When phosphenes are produced by excitation of the visual centers of the cerebral cortex, the subject ceases to see his surroundings and sees only light spots, which move with the eye movements. The excitation of adjacent regions of the cortex produce phosphenes of geometric and other shapes.

Phosphenes are not seen by persons blind from birth, but they may be induced in individuals who have become blind. Therefore, attempts are being made to create visual prostheses using artificial excitation of phosphenes. Phosphenes seen by a person in the presence of illumination sometimes blend with the images he sees of the real world, thereby creating visual illusions. Bright phosphenes may be a symptom of disease.