How the Brain Sees 3D Motion

Using fMRI (functional magnetic resonance imaging) researchers have now pinpointed where and how the brain processes 3-D motion. Surprisingly, findings published in Nature Neuroscience online July 7, 20091, reveal 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, known as MT+ (for Middle Temporal area), and its underlying neuron circuitry are so well studied that until now, most scientists had concluded that 3-D motion must be processed elsewhere. This is the first study that clearly links the area to 3D motion perception.

Critical Survival Mechanism

Ducking the head-on lunge of a lion or a thrown rock calls upon the power of the human brain to process 3-D motion. Humans have frontal facing vision, enabling stereopsis, in which parallax provided by the two eyes’ different positions on the head give precise depth perception2. That makes the ability to rapidly recognize an object moving in three dimensions critical to survival.

“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+,” according to Alexander Huk, assistant professor of neurobiology, Institute for Neuroscience at The University of Texas, Austin. “Given how much we already know about MT+, this research gives us strong clues about how the brain processes 3-D motion.”

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 showed that the MT+ area had high neural activity when participants looked at objects, in this case, small dots, moving toward and away from their eyes. Colorized images of participants’ brains show the MT+ area glowing bright blue.

Binocular Cue Encoding

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 parallax, or binocular disparity.

This can be seen when you alternate between closing your left and right eye; objects seems to shift back and forth. For moving objects, the brain calculates changes in this mismatch over time.

Concurrently, an object moving directly toward the eyes will move across the left eye’s retina from right to left, but moves across the right eye’s retina from left to right. The two ways of processing are summed together in the MT+ area.

“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.”

“Who cares if the tiger or the spear is going from side to side?” adds 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.”

References

1. Disparity- and velocity-based signals for three-dimensional motion perception in human MT+ Bas Rokers1, Lawrence K Cormack1 & Alexander C Huk Nature Neuroscience
Published online: 5 July 2009 | doi:10.1038/nn.2343
2. Wheatstone, C. (1838). Contributions to the physiology of vision.—Part the First. On some remarkable, and hitherto unobserved, phænomena of binocular vision. Philosophical Transactions of the Royal Society of London, 128, 371-394
3. Scott B. Steinman, Barbara A. Steinman and Ralph Philip Garzia. (2000). Foundations of Binocular Vision: A Clinical perspective. McGraw-Hill Medical. ISBN 0-8385-2670-5.