Dr. Bas Rokers investigates the brain's 3D-motion processing, the MT+ area. Image source: Marsha Miller

Ducking a punch calls for the human brain to process 3D motion; thus, perceiving an object that is moving in three dimensions is critical to survival, according to findings published online in Nature Neuroscience earlier this month.

Neuroscientists at the University of Texas (UT) at Austin said that they have now pinpointed where and how the brain processes 3D motion using specially developed computer displays and a functional MRI (fMRI) system to scan the brain.

The researchers were surprised to find that 3D 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 2D motion (up, down, left and right). This area, known as MT+, and its underlying neuron circuitry are so well studied that most scientists concluded that 3D motion must be processed elsewhere.

"Our research suggests that a large set of rich and important functions related to 3D motion perception may have been previously overlooked in MT+," said Alexander Huk, assistant professor of neurobiology at UT. "Given how much we already know about MT+, this research gives us strong clues about how the brain processes 3D motion."

For the study, Huk and his colleagues had people watch 3D visualizations while lying motionless for one to 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 (in this case, small dots) moving toward and away from their eyes. Colorized images of participants' brains show the MT+ area awash in bright blue, according to the authors. They wrote that the tests also revealed how the MT+ area processes 3D motion: it simultaneously encodes two types of cues coming from moving objects.

There is a mismatch between what the left and right eyes see, called a binocular disparity. For a moving object, the researchers said that 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 3D motion up," Huk said. "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.

"Who cares if the tiger or the spear is going from side to side?" said Lawrence Cormack, an associate professor of psychology at UT. "The most important kind of motion you can see is something coming at you, and this critical process has been elusive to us. Now we are beginning to understand where it occurs in the brain."

The researchers added that it also leads to a lot of fun at 3D movies.

The research was supported by a National Science Foundation CAREER Award to Huk.