The sight of a face offers the brain something special. More than a set of features, it conveys the emotions, intent, and identity of the whole individual. The same is not true for the body; cues such as posture convey some social information, but the image of a body does not substitute for a face.
A brain imaging study at the Rockefeller University offers some insight into how faces achieve this special status. The scientists found that certain spots dedicated to processing faces in the primate brain prefer faces with bodies–evidence they are combining both facial and body information to represent an individual.
The study, published on October 13 in the Proceedings of the National Academy of Sciences, was conducted in rhesus macaque monkeys. Humans have a similar system that responds to faces, suggesting the findings have relevance for understanding our own social processing as well.
“The body, arguably, is the most important contextual clue a viewer has to help make sense of a face,” says senior author Winrich Freiwald, an assistant professor and head of the Laboratory of Neural Systems. “Work by Clark Fisher, a graduate student in my lab, is remarkable in that it shows how the face-processing network places information about a face into its natural context as part of the body, and so begins to generate a sense of agency associated with the whole individual.”
How the brain processes faces and bodies
In work published in 2008, Freiwald and his colleague Doris Tsao showed that a network of patches along a deep groove in the sides of the macaque brain act as a specialized system for processing faces. A similar system has been found in the human brain, although it is not yet clear how the respective networks align. Both macaque and human brains also have separate patches that respond to bodies.
The conventional anatomical wisdom is that both species’ brains process faces and bodies independently. However, some studies of human perception suggest a more complex situation. For instance, one study found people’s perception of the emotion shown by a face can be altered by body posture, even when the viewers were told to disregard the body.
More than the sum of the parts
In the study, Fisher began by showing macaques still images that either displayed the face of a fellow macaque alone, the body without a face, or the entire animal.
Using high-resolution brain activity scans, captured with a method known as functional magnetic resonance imaging, he recorded how each of the six macaque facial patches, located in a part of the brain called the superior temporal sulcus (STS), responded. This approach was intended to reveal if a patch reacted strictly to faces or to some degree to bodies as well–or if a patch preferred a face and body together more than the sum of both presented separately.
“The only known way to get what we call a superadditive response, which exceeds those prompted by an individual face and body combined, is if there is some kind of interaction between the facial and body information in the patch,” Fisher says. This interaction is important because it suggests the brain is no longer just receiving information from the eyes, but beginning to make sense of it.
Two of the four face patches, one in particular, showed evidence of a superadditive response. When Fisher replaced the macaque bodies with images of other objects–a metronome, a spray bottle, a power tool–superadditivity disappeared from these patches. This result suggested the two patches were responding specifically to bodies, not just any object.
He also performed the same experiments while looking at two neighboring body patches, but these patches appeared largely uninterested in faces. This finding appears to match the asymmetry found in human social perception–the fact that bodies influence our perception of faces, while faces do not really add to our reading of bodies.
Aside from body context, another crucial clue to the state of mind and intent of another individual comes from the motion of his or her face. Previously, Fisher and Freiwald found face patches respond to facial motion. As it turns out, face patches’ preference for bodies and for facial motion intersect at one particular patch. Located within a region at the front of the STS, this patch responds strongly to both.
“We now think this particular face patch might be a critical node in social cognition, the process by which the brain infers a sense of agency for another individual and so determines how to interact appropriately,” Freiwald says.
About this neuroscience research
Source: Wynne Parry – Rockefeller University Image Credit: The image is credited to Laboratory of Neural Systems at The Rockefeller University Original Research:Abstract for “Whole-agent selectivity within the macaque face-processing system” by Clark Fisher and Winrich A. Freiwald in PNAS. Published online October 13 2015 doi:10.1073/pnas.1512378112
Whole-agent selectivity within the macaque face-processing system
The primate brain contains a set of face-selective areas, which are thought to extract the rich social information that faces provide, such as emotional state and personal identity. The nature of this information raises a fundamental question about these face-selective areas: Do they respond to a face purely because of its visual attributes, or because the face embodies a larger social agent? Here, we used functional magnetic resonance imaging to determine whether the macaque face patch system exhibits a whole-agent response above and beyond its responses to individually presented faces and bodies. We found a systematic development of whole-agent preference through the face patches, from subadditive integration of face and body responses in posterior face patches to superadditive integration in anterior face patches. Superadditivity was not observed for faces atop nonbody objects, implying categorical specificity of face–body interaction. Furthermore, superadditivity was robust to visual degradation of facial detail, suggesting whole-agent selectivity does not require prior face recognition. In contrast, even the body patches immediately adjacent to anterior face areas did not exhibit superadditivity. This asymmetry between face- and body-processing systems may explain why observers attribute bodies’ social signals to faces, and not vice versa. The development of whole-agent selectivity from posterior to anterior face patches, in concert with the recently described development of natural motion selectivity from ventral to dorsal face patches, identifies a single face patch, AF (anterior fundus), as a likely link between the analysis of facial shape and semantic inferences about other agents.
“Whole-agent selectivity within the macaque face-processing system” by Clark Fisher and Winrich A. Freiwald in PNAS. Published online October 13 2015 doi:10.1073/pnas.1512378112