The people who can see in pitch darknessS

Some people can "see" their own hands waving in front of their faces while in total darkness. It shouldn't be possible, but a quirk of their senses allows these people to perceive movement without using their eyes.

In 2005, Duje Tadin was working on a post-doc experiment with his mentor Randolph Blake, a psychologist who studies vision perception at Vanderbilt University in Tennessee. For their research, the scientists needed a "perfect blindfold," so they went online and purchased a black blindfold made of plastic.

"My mentor put it on and he tested it by waving his hand in front of his face, and what he had was a sensation of a moving image," said Tadin, who's now a cognitive scientist with the University of Rochester in New York. "Then I put it on my head, did the same thing and had the same sensation."

No light reached their eyes, but the researchers were somehow able to perceive the movement of their own hands in front of their faces. Why did they feel like they were seeing something when they obviously could see nothing? They wondered whether other people would also get the same visual sensation in the absence of visual input. They wanted to set up an experiment to test it. But that brought up a major problem: How do you objectively measure a subjective experience?

Answering that question became Tadin's goal. "If I just ask people if they see something, they might say yes just because I am asking them," Tadin told io9. "This [problem] has been bugging me for years."

A few years later, Tadin left Vanderbilt, joined the faculty in Rochester and set his first graduate student, Kevin Dieter, to the task of devising a robust experiment to test how many people can "see" their hands without using their eyes. The researchers created a clever experiment, which hinged upon expectations and deception. They coupled it with eye-tracking technology. The result? They discovered that some people can indeed see their hand movements in total darkness.

Almost a decade after their serendipitous discovery, Tadin, Blake and their colleagues have finally published their work showing that people really can visually perceive the movement of their own hands in total darkness. Here's how they did it.

Making the Invisible Visible

The researchers began by lying to study participants. They told subjects that they were investigating "visual sensitivity to motion under low lighting conditions." They showed participants two blindfolds — one normal, the other with several dozen hole-like indentations — and told them that one blindfold would block all light, while the other might allow tiny amounts of light to pass through. In actuality, however, both blindfolds blocked out all light.

The team then conducted two trials with the blindfolds. The participants randomly put on one of the blindfolds, but did not know which one they actually had (cardboard stickers prevented them from feeling any indentations as they put on the blindfolds). With the blindfold on, the participants moved one of their hands from side to side in front of their faces. While they did this, they answered a series of questions that probed if they had any visual sensations and how vivid those sensations were, such as if they saw movement or any discernable shapes. They then repeated the test with the other blindfold.

This experiment played on the participants' expectations. "If in the first trial you had a faint sensation, you'd think, 'The blindfold I got was the one with the holes,'" Tadin explained. "So if you know you are getting a different blindfold, you'd expect to see nothing at all in the second trial."

This means that if the participants couldn't actually see anything and were just answering according to how they thought they should, their reports should follow along their own biases. If they say they see something in the first trial, they should say they don't see anything in the second trial, and vice versa. But this isn't what happened.

Approximately 50 percent of the participants reported visual sensations in the first trial — most commonly they had a "visual sensation with motion," which they described as a moving shadow. Less than half of these people reported visual sensations again in the second trial, showing that biases did come into play a little. However, all but one person who reported seeing something in the second trial also reported sensations in the first trial, suggesting that the visual perceptions were real.

The researchers followed up with a second experiment that repeated the first, with just one alteration: They told the participants that they might get the same blindfold twice, negating any expectations. In this case, about 50 percent of the participants reported seeing something in both trials.

Though half of the participants said they saw something during the experiments, Tadin thinks the actual number may be more than that. "Afterwards when we debriefed them, we had a number of people who said they saw something but thought they were just imagining it," he said.

A Blending of the Senses

Tadin and his team suspected that this odd sight-without-light is the result of a kind of blending of the senses (kinesthetic and visual), where the actions of the body are somehow affecting how people perceived vision. "We realized that's essentially what synesthesia is," Tadin said. Synesthetes, or people with synesthesia, may taste numbers, smell words or hear colors, among other things.

So the team decided to repeat their first experiment with a group of grapheme-color synesthetes — people who always associate letters and numbers with specific colors. The results were off the charts.

All nine of the synesthetes experienced visual perceptions in both trials, even though they were led to believe they wouldn't see anything in the second trial. What's more, the images the synesthetes saw were more defined than the images the non-synesthetes saw: Instead of perceiving a faint shadow, they described seeing something akin to an inverted pendulum, and some even said they saw fingers.

The team then performed eye-tracking experiments on synesthetes and non-synesthetes. Research has shown that you can only have smooth eye movements from side to side if you are tracking an object — if you're not locked on to an object, your eye movements will be jerky. This is actually why physicians instruct patients to follow their finger, Tadin explained.

"If a patient shows nice eye movement, then the visual system is working correctly," Tadin said. "If there's jerky eye movement, then there may be neurological problems."

Tadin and his colleagues put the participants in a totally dark room and fitted them with eye tracking cameras that use infrared light. They had the participants wave one of their hands in front of their faces again, and told them to try to track their hands as best they could. The participants who reported being able to see motion had eye movements that were over twice as smooth as those who couldn't see any motion. Unsurprisingly, the synesthetes did far better than the non-synesthetes

"We had one synesthete who gave us perfectly smooth movement in darkness," Tadin said.

Importantly, the participants could only see their own hands. In both the blindfold and eye-tracking experiments, the participants couldn't see the movement of another person's hand.

The research suggests that our sensory systems really work together to create our perceptions. The visual system appears to take input from your movements to "predict" what you're about to see, Tadin said, adding that this ability may function to improve hand-eye coordination.

It's not clear why some people could not "see" their hands in the dark. "There's clearly something different in the brains of these people," Tadin said. "You can speculate that maybe they have poorer hand-eye coordination."

The researchers are now looking to do a similar study using older adults, hypothesizing that they would do worse on the tasks because your hand-eye coordination decreases as you age (you have less connectivity between your senses). They also want to conduct experiments in the light to try to understand how the brain creates this crosstalk between the senses.

"The idea is to see how this works in natural vision, because most of us don't walk around with a blindfold on," Tadin said. " How does the brain combine the action sense with the visual sense?"

Check out the study in the journal Psychological Science.

Top image via J. Adam Fenster / University of Rochester. Videos via Kevin Dieter & University of Rochester.