Just in time for Halloween, researchers suggested those scary shadows you see in a dark room may actually be your brain interpreting your own physical movements.
A research team used eye tracking technology on129 people walking in the pitch black, and found 50 percent of them could still see the movement of their own hand, a Vanderbilt University news release reported.
"Seeing in total darkness? According to the current understanding of natural vision, that just doesn't happen," Duje Tadin, a professor of brain and cognitive sciences at the University of Rochester who led the investigation, said. "This research shows that our own movements transmit sensory signals that also can create real visual perceptions in the brain, even in the complete absence of optical input."
The study provides clues as to how the human brain processes sensory information.
"Any time you willfully execute a movement-such as waving your hand in front of your face-your brain generates command signals sent to the muscles causing them to produce the movement. Having issued those motor orders, the brain also expects them to be carried out, and that expectation is signaled to other parts of the brain as a heads-up that something is about to happen," Randolph Blake, Centennial Professor of Psychology at Vanderbilt University and co-author of the study, said. "We surmise that those heads-up signals find their way into the visual pathways, thus producing an illusory impression of what would ordinarily be seen-a kind of self-fulfilling prophecy."
The idea for the study was discovered by accident when the researchers were performing a different experiment that required a blindfold.
"Just for fun I put on the blindfold and waved my hand in front of my face. I was astonished to see that that hand motion remained faintly visible to me," Blake said.
This may confirm the existence of the "spelunker illusion," which occurs when explorers in completely dark caves can still see their hands.
The researchers believe this ability could be learned over time, instead of being an innate trait.
"We get such reliable exposure to the sight of our own hand moving that our brains learn to predict the expected moving image even without actual visual input," Vanderbilt postdoctoral researcher Kevin Dieter, said.
One group of study participants was told they would be able to see shadows, and the blindfolds appeared to have small holes in them, this was false. The other group was told they would not be able to see at all. A third group had the experimenter wave their hand in front of the subject's face instead of the participant doing it for themselves.
The researchers also tested people who had a condition called synesthetes, which caused them to mix up certain senses; the individuals in this study saw numbers and letters in specific colors.
Half of the entire study found they were able to detect their hand every time, but very few were able to see the experimenter waving their hand. Those with synesthetes were more successful in seeing movement and form.
One subject with synesthetes was able to track her hand with 95 percent accuracy, which was almost equivalent to the accuracy one would expect if the lights were on.
"We know that sensory cross-talk underlies synesthesia. But seeing color with numbers is probably just the tip of the iceberg. Brains are wired for connectivity," Tadin said.