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How primates quickly detect faces

Researchers are gaining insights into the brain circuits involved in detecting faces.Johnathan M. Thomas / Shutterstock

The brains of primates, including monkeys, apes, and people, can pick out the faces of their fellow species members at astonishing speeds, even when they’re in their peripheral vision. But how the primate brain performs this feat isn’t entirely understood.

Groups of neurons called “face patches,” which are scattered throughout various parts of the cerebral cortex and nearby brain regions, play a role in processing faces and their expressions. They do this relatively quickly, in less than a second. But these patches don’t explain the speed with which primate brains first recognize the presence of a face, regardless of its identity. It’s also known that babies generally take special interest in faces even before face patches develop in their brains.

A group of NIH researchers led by Dr. Richard Krauzlis has been looking at whether neurons in an area deeper in the brain, called the superior colliculus (SC), play a role in rapid face processing. In a new study, they tested the response speed of neurons in the SC in rhesus monkeys to a variety of visual stimuli, including faces. Their results were published on June 27, 2024, in Neuron.

The team assembled a collection of images, including faces, other body parts like hands and arms, and items like fruit or human-made objects. They then showed these images in the peripheral visual field of adult monkeys and recorded neuronal responses in the SC.

They found that, within just 40 milliseconds, more than half the neurons they measured in the SC responded preferentially to images of faces compared to other types of objects. Some neurons in the area eventually picked up other types of objects, but not until 80 to 100 milliseconds. These results show that the vision circuits in the SC are much better at detecting faces than other objects.

The team next performed experiments to understand the route by which this information travels from the eyes to the SC. When they temporarily blocked activity of neurons in an area of the brain called the lateral geniculate nucleus, which relays visual signals to the cortex, the rapid responses to faces and other visual stimuli was lost. This indicates that a route through the visual cortex, rather than signals coming directly from the eye to the SC, is vital for the rapid response to faces.

“This newly discovered circuit explains how we’re able to quickly detect and look at faces, even if they first show up in the peripheral visual field where visual acuity is poor,” Krauzlis says. “This circuit could be what spotlights faces to help the brain learn to recognize individuals and understand complex facial expressions, helping us acquire important social interaction skills.”

More work will be needed to determine the exact neuronal route that gives rise to face detection in the SC. These findings could have implications for understanding conditions such as autism, where face detection and recognition are often impaired from early childhood.