From Wikipedia:
The dreamachine (or dream machine) is a stroboscopic flicker device that produces visual stimuli. A dreamachine is "viewed" with the eyes closed: the pulsating light stimulates the optical nerve and alters the brain's electrical oscillations. The "viewer" experiences increasingly bright, complex patterns of color behind their closed eyelids. The patterns become shapes and symbols, swirling around, until the "viewer" feels surrounded by colors. It is claimed that viewing a dreamachine allows one to enter a hypnagogic state. This experience may sometimes be quite intense, but to escape from it, one needs only to open one's eyes.

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