Researchers led by Professor Greg DeAngelis of the University of Rochester noted when we use two eyes, the brain calculates the depth or distance of an object from the slight differences in the two images of the object by a phenomenon called "binocular disparity."

Although scientists also knew people could judge distance using just one eye, the mechanism by which the human brain accomplished that feat has been unknown.

DeAngelis said neurons in the middle temporal area of the brain calculate distance by combining visual information from the eye with information about the physical movement of the eye.

"It looks as though in this area of the brain, the neurons are combining visual cues and non-visual cues to come up with a unique way to determine depth," said DeAngelis. He said the brain, when tracking the movement of a group of objects, use middle temporal neurons to calculate the object moving fastest in the same direction must be the closest object and the one moving slowest must be the farthest.

The study's findings appear in the online issue of the journal Nature.