Ganzfeld effect
The Ganzfeld effect (from German for "complete field"), or perceptual deprivation, is a phenomenon of perception caused by exposure to an unstructured, uniform stimulation 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. It has been most studied with vision by staring at an undifferentiated and uniform field of color. The visual effect is described as the loss of vision as the brain cuts off the unchanging signal from the eyes. The  result is "seeing black", an apparent sense of blindness. A flickering ganzfeld causes geometrical patterns and colors to appear, and this is the working principle for mind machines and the Dreamachine. The ganzfeld effect can also elicit hallucinatory percepts in many people, in addition to an altered state of consciousness. Ganzfeld induction in multiple senses is called multi-modal ganzfeld. This is usually done by wearing ganzfeld goggles in addition to headphones with a uniform stimulus. A related effect is sensory deprivation, although in this case a stimulus is minimized rather than unstructured. Hallucinations that appear under prolonged sensory deprivation  are similar to elementary percepts caused by luminous ganzfeld, and include transient sensations of light flashes or colours. Hallucinations caused by sensory deprivation can, like ganzfeld-induced hallucinations, turn into complex scenes. William G. Braud with Charles Honorton were the first to modify the ganzfeld procedure for parapsychological use. The effect is a component of the Ganzfeld experiment, a technique used in the field of parapsychology. History. In the 1930s, research by psychologist Wolfgang Metzger established that when subjects gazed into a featureless field of vision they consistently hallucinated and their electroencephalograms changed. The Ganzfeld effect has been reported since  ancient times. The adepts of Pythagoras retreated to pitch-black caves to receive wisdom through their visions, known as the prisoner's cinema. 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.