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TEXAS-AUSTIN (US)—A search to pinpoint the brain region responsible for processing 3D motion has, surprisingly, led neuroscientists to familiar territory—the same region long thought to process only two-dimensional motion.

Located just behind the left and right ears and known simply as MT+, the region and its underlying neuron circuitry are so well studied that most scientists had 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+,” says Alexander Huk, assistant professor of neurobiology at the University of Texas at Austin. “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 used specially developed computer displays and an fMRI (functional magnetic resonance imaging) machine to scan the brain while people watch 3D visualizations. 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.

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. 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 3D 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.

“Who cares if the tiger or the spear is going from side to side?” says Lawrence Cormack, associate professor of psychology. “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.”

Huk, Cormack, and postdoctoral research and lead author Bas Rokers published their findings in Nature Neuroscience online the week of July 7.

University of Texas at Austin news: www.utexas.edu/news