3D Visual Processing Centre Identified
Detecting 3D is a trait of stereoscopy. Its why humans tend to have two eyes, two different viewpoints create a degree of parallax, and allow us to perceive in 3D. However, motion parallax and movement in all three dimensions is harder to discern than simple 3D structure.
Impact avoidance depends on 3D motion detection, and it's a necessary survival trait. Thus, it has been long believed that there must be dedicated brain regions. After all, its exactly the skill needed to avoid a predator's jaws, dodge a punch, or help ensure your own punch connects. With the latter, the cerebellum controls the actual limb, but 3D motion detection confirms its course.
Recently, this area was finally detected, thanks to fMRI studies. Neuroscientists Dr. Bas Rokers, Alex Huk and Larry Cormac found the area, lighting up when 3D motion scans were carried out, and only triggered on three dimensional motion. They developed a new method of displaying the brain in three dimensions to make it easier to determine.
They 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).
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. "Given how much we already know about MT+, this research gives us strong clues about how the brain processes 3-D motion."
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.
"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."