October 31st, 2013
12:04 PM ET
Wave your hand slowly in front of your face.
Did your eyes track the movement? If so, your brain has formed a memory of that action; it will remember what the motion looks like in case you ever do it again.
In fact, a new study suggests that even if you wave your hand in front of your face in total darkness, your eyes may "see" it simply because they've seen it before.
"One thing our brains are exceptionally good at is picking up on reliable patterns," said lead study author Duje Tadin, professor of cognitive sciences at the University of Rochester. "Think about how many times you moved your hand and saw that movement ... It makes sense that our brains exploit this strong link."
Tadin and his colleagues conducted five experiments involving a total of 129 people. Their results were published online this week in the journal Psychological Science.
For the first two experiments, participants were shown two blindfolds. They were told that one of the blindfolds "may allow a small amount of light to pass through." In reality, both blindfolds blocked all light.
In other words, participants were led to expect NOT to see with one blindfold, and to expect to MAYBE see something with the other. This produced something called "experimentally controlled expectations" so researchers could collect data in a less subjective atmosphere.
Participants were asked to wave a hand over their eyes at a slow, comfortable pace while wearing one of the two blindfolds. Researchers then asked them whether they saw anything, and if they did, what they saw.
In the first experiment, participants were told both blindfolds would be used, so they expected to potentially see in one of the two trials. In the second experiment, participants were told they had a 50/50 chance of getting the "seeing" blindfold each time.
About 50% of the participants reported experiencing some visual sensation in the first two experiments, despite being completely blindfolded; a few described well-defined forms in the darkness. Others confidently reported not seeing anything at all.
The researchers concluded that kinesthesis, or movement in your limbs, can actually generate visual sensations - but it's a learned ability.
"Our brain predicts what we are about to see and generates an image accordingly," Tadin said.
I can't see you
In the third experiment, the researcher waved his or her hand in front of the participant's face while they were blindfolded.
In the first trial, no one reported seeing the researcher's hand. In the second, only two of 16 participants reported seeing movement.
This supported the researchers' theory that you can only "see" your own body motion because your brain has so many reliable memories of its own limbs.
The super seers
In the fourth experiment, a select group of users with synesthesia replicated the first experiment. People with synesthesia experience a rare blending of the senses; some see colors when they read, others experience tastes when they speak certain phrases or words.
"What we initially discovered was a blending of the senses - our subjects had visual sensations that were caused by another sense," Tadin said. "We hypothesized that this ability to see your own hand in darkness would be stronger in synesthetes."
The synesthetes' (people who experience synesthesia) results were "literally off the chart," Tadin said; they all reported very vivid sensations of seeing their hand in motion while blindfolded.
More objective data
In the fifth experiment, researchers removed the blindfolds and placed participants in total darkness with a head-mounted eye tracker. The machine recorded their eye movement as they first waved their own hand in front of their face, then had a researcher wave his/her hand, and then waved a cardboard cutout of a hand.
This experiment provided objective evidence to back up the other experiments' findings. People who reported seeing their hand had eye movements that were twice as smooth as those who did not.
"We can only generate smooth following eye movements if we have a moving target to lock on," Tadin explained. "We had one synesthete that had perfectly smooth eye movements when following her hand in total darkness. For her, we really thought we made a mistake and left the lights on."
This doesn't mean that humans are preprogrammed to see in the dark. It's learned, Tadin said, and likely only applies to our own body movement.
Tadin and his colleagues hope this research can one day be used to improve hand-eye coordination. They are currently working on this idea with a group of older adults, a population that often experiences hand-eye coordination difficulties.
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Get a behind-the-scenes look at the latest stories from CNN Chief Medical Correspondent, Dr. Sanjay Gupta, Senior Medical Correspondent Elizabeth Cohen and the CNN Medical Unit producers. They'll share news and views on health and medical trends - info that will help you take better care of yourself and the people you love.