A new study by Israeli and French researchers says it has managed to identify, for the first time, neurons in the human visual cortex that can selectively respond to faces.
“A lot of research has been done on humans” regarding how they process faces, said Vadim Axelrod, head of the Consciousness and Cognition Laboratory at the Gonda (Goldschmied) Multidisciplinary Brain Research Center at Bar-Ilan University, who carried out the study. “But this is the first time we recorded human neurons that respond to faces in the visual cortex,” he said in a phone interview with The Times of Israel.
The research “is an important step in better understanding the mechanism of how a face is processed in the human brain” at a cellular level, Axelrod said. The finding “can potentially help understand how people recognize faces.”
Axelrod worked in collaboration with a team from Institut du Cerveau et de la Moelle Épinière and Pitié-Salpêtrière Hospital in Paris, led by Prof. Lionel Naccache. The study was published in the January 22 issue of Neurology, the medical journal of the American Academy of Neurology.
“Recognizing faces is central to the cognition of human beings,” Axelrod said. All of our social interaction is based on our ability to process faces and their differences: it allows us recognize people we know even when we haven’t seen them in a long time, identify celebrities, express our feelings, discern trust and more.
In their study, the researchers showed that the neurons in the visual cortex responded much more strongly to faces than to cityscapes or objects. The neurons are located in the posterior temporal visual cortex, in the vicinity of the Fusiform face area, lately one of the most extensively studied brain areas and the largest and likely the most important in face selection, the researchers said in a statement announcing their study.
A high neural response was found both for faces of famous people like Charles Aznavour, Nicolas Sarkozy, Catherine Deneuve, and Louis De Funes, and for faces unfamiliar to the participant in the experiment. In an additional experiment, the neurons showed they could distinguish between human and animal faces that appeared within a movie — a clip from Charlie Chaplin’s “The Circus.”
“In the early 1970s, Prof. Charles Gross and colleagues discovered the neurons in the visual cortex of macaque monkeys that responded to faces,” Axelrod said in the statement. “In humans, face-selective activity has been extensively investigated, mainly using non-invasive tools such as functional magnetic resonance imaging (fMRI) and electrophysiology (EEG).”
“Strikingly,” he added, face-neurons in the posterior temporal visual cortex “have never been identified before in humans. In our study, we had a very rare opportunity to record neural activity in a single patient while micro-electrodes were implanted in the vicinity of the Fusiform Face Area.”
The researchers stumbled upon the neurons while studying a patient who had epilepsy, said Axelrod. “We put the electrodes close to that area,” he said. “We were lucky.”
Up until now, the only neuron that have been found to respond to faces have been the so-called “Jennifer Aniston cells” − neurons located in the medial temporal lobe of the brain that respond to different images of a specific person, in this case Jennifer Aniston, in an original study published in Nature by Rodrigo Quiroga and his colleagues in 2005.
The researchers in the Quiroga team found that these cells fire up only in response to pictures of specific people — like Jennifer Aniston, Bill Clinton, or the Beatles. These neurons were said to respond to the concept of the person, even a drawing, a picture or a masked image, the researchers said.
The neurons in the visual cortex that were identified by the Israeli and French researchers “are very different” from the neurons found by the Quiroga team, Axelrod said, because they “respond vigorously to any type of face, regardless of the person’s identity.”
In addition, these newfound neurons respond much earlier, he said.
“While in our case, a strong response could be observed within 150 milliseconds of showing the image,” he said. “The ‘Jennifer Aniston cells’ usually take 300 milliseconds or more to respond.”
These findings can also help bridge the understanding of face mechanisms across species, like in monkeys and humans. “It is really exciting,” Axelrod said, “that after almost half a century since the discovery of face-neurons in macaque monkeys, it is now possible to demonstrate similar neurons in humans.”