When it comes to seeing in the dark, traditional belief is that humans are not able to. However, new research has challenged this, suggesting that at least 50% of all people are able to see the movement of their own hand, even in an environment that is pitch black.
Researchers from the University of Rochester in New York and Vanderbilt University in Tennessee say their findings suggest that what humans normally understand as sight has just as much to do with the brain as it does the eyes.
Duje Tadin, professor of brain and cognitive sciences at the University of Rochester, and Randolph Blake, centennial professor of psychology at Vanderbilt University, say they first discovered that they were able to see their own hand movement in the dark in 2005.
But for the study, published in the journal Psychological Science, the researchers say the challenge was to devise experiments that objectively measured a perception that was subjective.
"While the phenomenon looked real to us, how could we determine if other people were really seeing their own moving hand rather than just telling us what they thought we wanted to hear?," says Prof. Blake.
Kevin Dieter, a postdoctoral fellow at Vanderbilt University, came to their aid as he helped create experiments with the aim of leading the participants to accurately report what they were visualizing.
Experiments under 'false expectations'
The researchers conducted a series of experiments on 129 participants from Rochester, NY, Nashville, TN, Fenton, MI, and Seoul in South Korea.
Some of these participants were chosen because they experience a "blend of senses" in their daily life. They are known as "synesthetes."
The researchers say this means they may see colors when they hear music, or have the ability to experience taste when they hear sounds. For this study, the synesthetes involved were able to see numbers or letters in specific colors.
One experiment required the participants to wear blindfolds that appeared to have lots of holes. The participants were then told they would see "motion under low lighting conditions."
Another experiment required the same participants to wear similar blindfolds without the holes, and the researchers led them to believe they would see nothing.
However, the researchers say that in both experiments, the blindfolds did not have holes and were blocking out all light. The purpose of telling the participants otherwise was to create "false expectations" so they would truly report what they were seeing in the coming experiments.
During these experiments, one of the researchers waved his hand in front of the blindfolded participant, and the participants were also required to move their own hands in front of the blindfolds.
The subjects' eye movements were monitored with a computerized eye tracker in complete darkness in order to find out whether the participants would report the ability to see hand movements.
Self-movement important to visual process
The researchers found that across all participants, approximately 50% were able to detect their own hand movements consistently, regardless of the false expectations created.
However, when an experimenter waved his hand in front of the participants, the subjects reported seeing no movement. The researchers say this highlights the importance of "self-movement" in this visual process.
The video below from the University of Rochester explains the basis of the experiments:
By using the eye tracker, the researchers also found that the participants who were able to see the motion of their hand had the ability to track their movement much more accurately in the dark, compared with those who reported no visual sensation.
Commenting on these findings, Prof. Tadin says:
"Seeing in total darkness? According to the current understanding of natural vision, that just doesn't happen.
But 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."
Potential neural connections with self-motion
When looking at the synesthetes, the researchers found that they had a better ability to see movement, as well as seeing a clearer visual form of that movement.
The researchers say that one synesthete even reported 95% accuracy on following the movement of their hand in darkness, meaning they could see their hand in complete darkness almost as well as in daylight.
These findings have led the researchers to believe that the link with synesthesia suggests the ability to see self-motion in the dark may be associated with neural connections between senses.
"We know that sensory cross talk underlies synesthesia. But seeing color with numbers is probably just the tip of the iceberg. Synesthesia may involve many areas of atypical brain processing," says David Knill, professor of brain and cognitive sciences at Rochester University.
But Prof. Tadin says it is unlikely that the majority of humans are "preprogrammed" to have the ability to see in the dark.
"Innate or experience? I'm pretty sure it's experience. Our brains are remarkably good at finding such reliable patterns," he says.
"The brain is there to pick up patterns - visual, auditory, thinking, movement. And this is one association that is so highly repeatable that it is logical our brains picked up on it and exploited it."