[1] The effect is the result of the brain amplifying neural noise in order to look for the missing visual signals.
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.
[6] 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.
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.