Phosphenes

Phosphenes are apparent flashes of of light that appear without any light coming into the eye. They can occur when the eye is stimulated by mechanical pressure on the eyeball, with electrical stimulation of the visual cortex, or just by means of random firing of cells in the neural system.



Here's an explanation (Link to video), along with a way to demonstrate the blind spot illusion.

Phosphenes can be experienced by people who have been blind from birth when their brains are stimulated directly, or by people confined for long periods in darkness (where the effect is sometimes called "prisoner's cinema"), and by people experiencing hallucinogenic drugs.
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Wikipedia on Phosphenes