How do phosphenes (light shows behind the eyelids) happen?

Everybody knows these free psychedelic color effects. A little gentle pressure on the eyeball is all it takes. But what causes them? What kind of neural firing happens to make these light shows?

Cecil knows.

I make 'em by closing my eyes and kind banging my eyelids (not too hard!) with the tips of my fingers! :smiley:

You can also generate phosphemes by directly stimulating the optic nerve with an electrical current of a few volts. A wave form generator or muscle stimulator that can produce square waves from 1-50 Hz works pretty well for this. Simply hook the electrodes up to the right locations around your eyeballs, turn a few knobs and enjoy the show !
If you want to experiment with a phospheme generator, Scientific American ran an article describing phosphemes and phospheme generators in the late 60’s or early 70’s. It shouldn’t be too hard to find at the local library.