The Ganzfeld effect is a phenomenon of visual perception caused by staring at an undifferentiated and uniform field of color. The effect is described as the loss of vision as the brain cuts off the unchanging signal from the eyes. The result is “seeing black” or apparent blindness. The word is from German for “complete field”.
The effect is the result of the brain amplifying neural noise in order to look for the missing visual signals. The noise is interpreted in the higher visual cortex, and gives rise to hallucinations. This is similar to dream production because of the brain’s state of sensory deprivation during sleep.
The Ganzfeld effect has been reported since ancient times. The adepts of Pythagoras retreated to pitch black caves to receive wisdom through their visions. Miners trapped by accidents in mines frequently reported hallucinations, visions and seeing ghosts when they were in the pitch dark for days. Arctic explorers seeing nothing but featureless landscape of white snow for a long time also reported hallucinations and an altered state of mind.
In Tibetan Buddhism a dark retreat refers to advanced practices of isolation in darkness. The time period dedicated to dark retreat varies from a few hours to decades. Dark retreat in the Himalayan tradition is a restricted practice only to be engaged by the senior spiritual practitioner under appropriate spiritual guidance. This practice is considered conducive for navigating the bardo at the time of death and for realising the rainbow body. The traditional dark retreat requires stability in the natural state and is only suitable for advanced practitioners.
Ancient Egyptians and Mayans practiced a form of the dark retreat as well, traditionally lasting 10 days. Holy men would enter into the center of their respective pyramids, completely removed from light and sound, and have visions of the workings of the universe. Today, scientists have hypothesized that when the human body is deprived of visual stimulation, the brain produces a substance called Dimethyltryptamine (DMT), a tryptamine, which results in intense visions.