The STS: A brain region for perceiving the intentions of others

Psychologists and neuroscientists can be said to be working on the same problem, but they tend to approach it from opposite directions. Psychologists generally look at behavior and then try to understand the mental processes that might cause that behavior. Neuroscientists look at brain activity and see how it corresponds to behavior. Then people who are considerably smarter than me try to synthesize the work of both psychologists and neuroscientists in order to come to a complete understanding of how we think and behave.

One piece to add this puzzle is a recent study by Kevin Pelphrey, James Morris, and Gregory McCarthy, a group of neuroscientists affiliated with the Brain Imaging and Analysis Center at Duke University ("Grasping the Intentions of Others: The Perceived Intentionality of an Action Influences Activie in the Superior Temporal Sulcus During Social Perception," Journal of Cognitive Neuroscience, 2004). They wanted to know what our brain is doing when we watch someone else do something. Are we simply reacting to what they do, or does our brain work differently when someone does something we don't expect?

They paid a group of young adults to submit to an MRI of their brains while watching an animated video display which showed two simple events. Each event depicted a man sitting at a workbench, which was empty except for a blue dial to the man's left. After a few seconds, the dial began to flash, and the man reached for it with his right hand, briefly adjusted it, and returned to his original position. This is exactly what you might expect someone to do in such a situation. In the second event, instead of reaching for the dial with his right hand, the man extended his left hand with the exact same motion, adjusting a nonexistant dial to his right, and returning again to the original position. This motion, while not particularly unusual, is certainly unexpected.

So, what happens to our brains when we perceive such a motion? Several different regions corresponding to vision are activated, all of which have lovely latin names like the occipito-temporal sulcus/fusiform gyrus and the parieto-temporo-occipital fossa. However, the area that attracted special interest in this study was the superior temporal sulcis, or STS. It's the only area which was activated only on one side of the brain, which was predicted based on other studies involving biological motion. Here's a picture showing the combined results for all conditions of the experiment:

It's important to recognize that we're not looking at a picture of the entire brain here: this is a top view of a thin "slice" of the brain; the area corresponding to the STS is labelled. What the MRI shows us is that the STS is active in both conditions of the experiment. Additional analysis, however, found that the STS was more active during the "incorrect" trials, when the animated figure reached in the wrong direction to adjust the dial. This makes the STS different from all regions of the brain except one, the MFG, which also displayed more activity during the "incorrect" trials. This region has previously been determined to be a center for social cognition, so this result was perhaps less surprising than the STS activity.

In previous experiments, Pelphrey and his colleagues had found that the STS was also important in detecting eye-gaze direction (see this Cognitive Daily article for more on gaze direction). This experiment allowed them to reason that the STS is not simply a center for detecting gaze direction, but a locus of social perception. So it appears that social perception is deeply embedded in our brains, not only in known areas for social cognition, but also in other areas devoted to other types of perception.