Pressing your palms against closed eyes produces a light show without a single photon entering the eye. These lights are phosphenes, from the Greek for to show light. This episode explains pressure phosphenes, including the shifting grid one scientist compared to a crumpling, fly-spotted fly screen, and how squeezing retinal ganglion cells makes them fire signals the brain can only read as light. It covers Isaac Newton sliding a blunt bodkin behind his own eyeball, 1800s French physicians pressing on blind eyes before cataract surgery to confirm the retina still worked, the prisoner’s cinema reported in total darkness, and the heart drug ivabradine, which can cause phosphenes as a side effect.

In optic neuritis, damaged nerve insulation lets a loud sound leak into the optic nerve and appear as a flash of light. Researchers in the late 1980s proposed that abstract Paleolithic cave art records phosphene form constants. In 1968 electrodes placed on a blind woman’s visual cortex produced points and bars of light, and early 1970s work used such grids to spell Braille. Later turns include astronauts seeing flashes from cosmic radiation, radiotherapy patients seeing blue Cherenkov light inside the eye, and the 2019 BrainNet experiment in which three people in separate rooms played a Tetris-style game linked brain to brain.

• Why stimulating tissue above the calcarine fissure produces light in the lower half of vision
• Transcranial magnetic stimulation and the magnetophosphenes it creates
• The debate over whether tACS stimulates the brain or leaks current to the retina
• Camera-fed bionic eye implants that turn the world into a grid of phosphenes
• BrainNet’s 15 and 17 hertz lights, EEG caps, a TMS helmet and an 81 percent success rate