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See Inside Scientific American Mind Volume 25, Issue 3

Synesthesia Helps Explain Mysterious Spelunker's Illusion

Why some people believe they can see their hands in total darkness



CHRIS GRAMLY Getty Images(Woman); ISTOCKPHOTO(eyes)

Many people swear by the so-called spelunker's illusion, in which they think they can see their own hands moving even in the total absence of light. You don't have to see it to believe it: in a recent article in Psychological Science, cognitive scientists based at Vanderbilt University and the University of Rochester have demonstrated that this spooky illusion is real, and some individuals are more prone to these visions than others.

Through a series of five experiments, the researchers asked their 129 subjects to report visual sensation in total darkness. In the first four setups, subjects wore a blindfold to block all light. A subset of these participants claimed to see movement when they waved their own hand in front of their face but not when an experimenter waved his hand.

Why would only some people think they could see the motion? On the hunch that this illusion was created by intense connectivity among brain regions, the research team had included volunteers with a form of synesthesia, in which heightened brain connectivity causes letters and numbers to appear as certain colors. These subjects, they discovered, had even stronger visual reactions to their own hands moving in the darkness than did the other subjects.

Finally, the researchers decided to try out the experiment using eye-tracking headgear, again in complete darkness. The eye tracker revealed that the more vividly a subject reported seeing his or her own hand's motion, the smoother the eye movements were. That is, their eyes behaved as though they really could “see” and were locking onto an imaginary target. In reality the participant was anticipating the visual experience of his or her hand in space.

Taken together, the studies suggest that people with heightened connectivity between the senses possess a greater awareness of the body. The findings are also a reminder that “sight” is generated by your brain, not your eyes. “The brain may or may not use information your eyes provide,” says Rochester cognitive scientist Duje Tadin. Instead your brain uses the eyes' information selectively alongside familiar or predictable patterns—such as your hands' movements—to construct what you ultimately perceive.

This article was originally published with the title "The Spelunker's Illusion."

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