Patterns of fMRI activity dissociate overlapping functional brain areas that respond to biological motion

A swaying tree and a moving person activate distinctive areas of the brain's visual cortex, since recognizing people is essential for social interaction. So, an important question in exploring the visual system is how the visual cortex manages such specific recognition of "biological motion."

In an article in the March 16, 2006, issue of Neuron, Marius Peelen and colleagues at the University of Wales, Bangor have used detailed functional magnetic resonance (fMRI) imaging of human volunteers to shed new light on this process. The widely used analytical technique of fMRI employs harmless magnetic fields and radio waves to measure blood flow in brain regions, which reflects brain activity in their region.

In their experiments, the researchers found that in detecting biological motion the visual cortex not only uses a specific region known to detect motion of other people, but engages areas that respond to the static human form as well.

The researchers designed their experiments to give subjects information only on biological motion and not on a human form. Specifically, they scanned the subjects' brains while showing them only "point-light" animation of human movement such as jumping or throwing. These animations consist only of a small number of white dots on a black background, not portraying skin, clothes, or other specific features of a human in action.

As a control, to separate effects of viewing motion in general, the researchers also showed the subject scrambled dots making the same motion, but not organized to create an illusion of a human in motion. The researchers also established the "body-selective" regions involved in recognizing the human form by showing subjects images of human faces and human bodies. And they mapped the region responsible for detecting motion in general by showing the subjects abstract oscillating patterns of rings.

Detailed analyses of the brain regions activated by the patterns revealed that the animations did activate the brain region responsible for detecting biological motion.

But importantly, the researchers also reported that "Our results show that visual areas involved in analyzing the form of the human body . . . are selectively activated by sparse movement patterns that induce the percept of a person performing an action." They also found that overlapping regions could nevertheless be responsible for distinctive components of the process.

"This work significantly clarifies our emerging picture of how the human brain makes sense of the appearance and actions of other individuals," concluded the researchers.

"We believe this approach will prove useful both in further studies on the neural basis of 'social vision' and more generally in studies of other regions where multiple functional areas occupy overlapping cortical territory," they wrote.