Brain enables sight without light

Many people can vaguely see their own hands moving in total darkness, thanks to brains that pick up slack for the eyes.

Sight without light is no figment of the imagination, say psychologist Kevin Dieter of the University of Rochester in New York and his colleagues. A person’s movements transmit sensory signals that the brain turns into visual perceptions of that motion, even if the eyes see nothing, the researchers propose October 30 in Psychological Science. The brain learns to associate the sight of one’s own hands in motion with the bodily sensations accompanying that activity.

The findings back up cave explorers’ accounts of having seen their hands as they scrambled through lightless underground spaces.

“The brain, through its action sense, ‘knows’ where a moving hand is and, as our results show, it actually generates the expected visual image,” says University of Rochester psychologist and study coauthor Duje Tadin.

But some people see nothing while waving a hand in the dark. The researchers suspect this lack of vision stems from insufficient cross talk between the brain’s vision and motion areas.

The new paper describes experiments with a total of 129 participants. In one trial, volunteers waved their hands in front of their faces while wearing blindfolds that blocked all light. An experimenter led them to believe that a small amount of light might get through the blindfolds, enabling minimal vision. In a second blindfold trial, volunteers believed they would see nothing.

Almost half of these individuals reported seeing motions while moving their hands, regardless of what they had been told to expect. Two people said they saw distinct hand outlines.

Few blindfolded participants reported seeing movements when an experimenter waved his hand in front of their faces.

Motion vision while blindfolded occurred most strongly among nine volunteers who regularly see particular numbers or letters in specific colors. These volunteers have synesthesia, a condition characterized by a blending of the senses. Although the causes of synesthesia are uncertain, the researchers hypothesize that extensive sensory cross talk prompted hand sightings in the dark by volunteers with synesthesia.

Computerized eye-tracking devices revealed that blindfolded volunteers who described seeing their hands also moved their eyes nearly as smoothly as if they were tracking objects in the light of day. Participants with synesthesia had particularly smooth eye movements. Simply imagining one’s hand moving would not produce this type of eye scanning, the researchers say.

But psychologist David Brang of Northwestern University in Evanston, Ill., disagrees. Some researchers have found that smooth eye tracking occurs when people imagine watching moving objects, he says, suggesting that sight without light partly depends on visualization, not brain cross talk.

There’s good evidence that people with synesthesia form especially clear mental images of objects, Brang adds. Thus, evidence that sensory cross talk underlies synesthesia is mixed, he says.