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Research at Vanderbilt

Even in the dark, brain “sees” its own body’s movement

by | Posted on Wednesday, Oct. 30, 2013 — 11:45 PM

On Halloween, the thought of walking through a dark room, hands outstretched to find your way, might take on a more sinister feel than usual, putting you on the lookout for shadowy figures. Yet new research indicates the shadows you see may not be hocus pocus, but your brain perceiving and interpreting your own movements in the dark.

With the help of computerized eye trackers, new research finds that at least 50 percent of people can see the movement of their own hand even in the absence of all light.

“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.”

Through five separate experiments testing 129 individuals, the authors found that this curious human ability to see motion provides clues to how the 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.”

New research from Randolph Blake, centennial professor of psychology, and postdoctoral researcher Kevin Dieter shows we can "see" our body's movements even in complete darkness. The researchers blindfolded research subjects and had them wave their hands in front of their faces and found they could actually see the movement in absence of light. (John Russell / Vanderbilt)

Tadin, Blake and colleagues at Vanderbilt and Rochester reported their findings online October 30 in Psychological Science, the flagship journal of the Association for Psychological Science.

“This project originated from an entirely accidental discovery Duje and I stumbled upon some years ago. We were doing an unrelated study that required the participant to wear a light-tight blindfold, and 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. “Duje then tried the same thing and saw the same thing. Being skeptical, we both moved to a completely dark, light-tight room, put on the blindfold and tried it again. Even though our eyes were receiving no light whatsoever we continued to see hand motion. That stunning but puzzling illusion prompted us to launch the project that culminated in this Psychological Science paper.”

The study seems to confirm anecdotal reports that spelunkers in lightless caves often are able to see their hands. In other words, the “spelunker illusion,” as one blogger dubbed it, is likely not an illusion after all.

Post-doctoral researcher Kevin Dieter demonstrates the conditions under which his colleagues discovered they could visually detect their hand's movement, even in the absence of light (John Russell / Vanderbilt)

For most people, this ability to see self-motion in darkness probably is learned, the authors conclude. “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. Dieter was the first author on the paper.

Dieter devised several ways to probe the intriguing observation. In one scenario, the researchers misled subjects by telling them to expect to see motion under low lighting conditions with blindfolds that appeared to have tiny holes in them. In a second setup, the same participants had similar blindfolds without the “holes” and were led to believe they would see nothing. In both setups, the blindfolds were, in fact, equally effective at blocking out all light. A third experiment consisted of the experimenter waving his hand in front of the blindfolded subject. Ultimately, participants were fitted with a computerized eye tracker in total darkness and asked to follow their hand as it passed in front of their face.

eye tracking device

Former study subject and UR graduate Lindsay Bronnenkant wears an eye-tracking device as she reenacts a study on synesthesia to illustrate new research that documents that many people have the ability to vaguely "see" the motion of their own body in complete darkness due to the connection between our brain's motion senses and our visual senses. (J. Adam Fenster / University of Rochester)

In addition to testing typical subjects, the team also recruited people who experience a blending of their senses in daily life. Known as synesthetes, these individuals may, for example, see colors when they hear music or even taste sounds. This study focused on grapheme-color synesthetes, individuals who always see numbers or letters in specific colors.

Across all types of participants, about half detected the motion of their own hand and they did so consistently, despite the expectations created with the faux holes. And very few subjects saw motion when the experimenter waved his hand, underscoring the importance of self-motion in this visual experience. As measured by the eye tracker, subjects who reported seeing motion were also able to smoothly track the motion of their hand in darkness more accurately than those who reported no visual sensation—46 percent versus 20 percent of the time.

Reports of the strength of visual images varied widely among participants, but synesthetes were strikingly better at not just seeing movement, but also experiencing clear visual form. As an extreme example in the eye tracking experiment, one synesthete exhibited near perfect smooth eye movement—95 percent accuracy—as she followed her hand in darkness. In other words, she could track her hand in total darkness as well as if the lights were on.

The link with synesthesia suggests that our human ability to see self-motion is likely based on neural connections between the senses, Tadin said. “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.”

Blake is an investigator in the Vanderbilt Kennedy Center for Research on Human Development. Paper co-authors also included David Knill and Bo Hu from the University of Rochester.

The study was funded by the following grants: NIH R01-EY019295 (to D.T.), R01-EY017939 (to D.K.), World Class University program through the Korea Science and Engineering Foundation funded by the Ministry of Education, Science and Technology (R31-10089 to R.B.), NIH P30-EY001319, P30-EY009126 and T32-EY007125.

Contact:
Melanie Moran, (615) 322-NEWS
melanie.moran@vanderbilt.edu


  • HarryObrian

    Tracking your hand is more accurate if you close your eyes… The publish or perish crowd is obviously running out of things to do…

  • Kabc Gabc

    I have a friend who is blind, and I’m sure he could have told you this. Spatial awareness is extremely important in sports, when you move before your brain has time to consciously decide to move. This is just another manifestation of that ability, muscle memory being closely related.

  • 808Townie

    Holy crap. I’ve “seen” this effect for years and years but never said anything to anyone because quite frankly – it sounds very much like the delusional arrogance that so many people, who are desperate to feel special, claim as a gift. I despise people like that. So that’s what I’ve seen. Interesting. And I’m glad it’s scientifically explained. More importantly, at 50%, is hardly “special”. That’s like claiming being male or female is special. Flip of a coin. Pretty standard. (I’m relieved) :-D

  • Jason Childress

    Have you ever heard of the Mind’s Eye? Science is so far behind, it’s comical.

  • disqus_M7byfgKxre

    Ask a guy who is laying on his back under a truck in the middle of winter trying to wrench off a bolt that he can’t see and can barely feel.
    Book learnin is gonna leave you a bit short there.

  • Howard Treesong

    Have you actually read the piece or do you only come to the intertubes to watch the purty pictures?

  • Lee J Rickard

    I have to ask: Why are so many of the responses to this article hostile?

  • krumr007

    This study does not seem too unrealistic but at the same time the mind can manipulate someone into thinking they see their hand just because they know their hand is moving. If this study was done with someone elses hand in a completely dark room and the results were the same it might be a little different because then people would take the results more seriously.

  • j_meissner

    How did you make sure that effect is not the contrary, to negate the visual impact of body movements? Like we see our nose all the time, but to make sure this is not a disadvantage, our brain deletes it out of our sight. What if our brain was able to predict body movements in our sight to make sure relevant sight would not be compromised?

  • geekofhearts

    yes, read the article. in the first version, with the blindfolds, one control group was someone else’s hand.

  • BrainMan

    Would they not continue to “see” it if their eyes were closed under the masks? What a scary example of “mob leap” into a non-scientific test

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