Neuroscientists have pinpointed where and how the brain processes 3-D motion.
They use specially developed computer displays and an fMRI (functional magnetic resonance imaging) machine to scan the brain.
The researchers found 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 (up, down, left and right).
The area is known simply as MT+.
"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.
"Given how much we already know about MT+, this research gives us strong clues about how the brain processes 3-D motion," he said.
To reach the conclusion, Huk and 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 (small dots) 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, when you alternate between closing your left and right eye, objects appear to jump back and forth.
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. It's seeing a change in position over time, and it's seeing opposite motions falling on the two retinas," Huk said.
That processing comes together in the MT+ area.