They found, surprisingly, that 3-D motion processing occurs in an area in the brain - located just behind the left and right ears - long thought to only be responsible for processing two-dimensional motion.

This area, known simply as MT+, and its underlying neuron circuitry are so well studied that most scientists had concluded that 3-D motion must be processed elsewhere. Until now.

"Our research suggests that a large set of rich and important functions related to 3-D motion perception may have been previously overlooked in MT+," says Alexander Huk, assistant professor of neurobiology at the University of Texas.

For the study, Huk and his colleagues had people watch 3-D visualizations while lying motionless for one or two hours in an MRI scanner fitted with a customized stereovision projection system.

The fMRI scans revealed that the MT+ area had intense neural activity when participants perceived objects moving toward and away from their eyes. Colorized images of participants' brains show the MT+ area awash in bright blue.

The tests also revealed how the MT+ area processes 3-D motion: it simultaneously encodes two types of cues coming from moving objects.

There is a mismatch between what the left and right eyes see. This is called binocular disparity. For a moving object, the brain calculates the change in this mismatch over time.

Simultaneously, an object speeding directly toward the eyes will move across the left eye's retina from right to left and the right eye's retina from left to right.

"The brain is using both of these ways to add 3-D motion up," says Huk. "It's seeing a change in position over time, and it's seeing opposite motions falling on the two retinas."That processing comes together in the MT+ area.; Source: The University of Texas at Austin