Despite intensive research on we can recognize the faces of friends and relatives we haven’t seen for years or celebrities we have never met, how the brain actually accomplishes this has remained a mystery.
Our world would be much different – and much poorer and more confusing – if everyone had the same face. Thanks to our parents’ genes handed down to us, everyone except for identical siblings look different and their faces express essential information. Unfortunately, among the demented, the ability to recognize faces of people they have known for decades is much impaired.
Most of us can recognize a famous person’s face even if it only appears for a fraction of second or the face of an old college friend even after decades of not seeing him. Many of us can sense the mood of a significant other just based on facial expression. Often, we can establish whether a person is trustworthy by just looking at his or her face.
A study just published in Neurology, the medical journal of the American Academy of Neurology, identifies for the first time the neurons in the human visual cortex that selectively respond to faces. The study was carried out by Dr. Vadim Axelrod, head of the Consciousness and Cognition Laboratory at the Gonda (Goldschmied) Multidisciplinary Brain Research Center at Bar-Ilan University, in collaboration with a team from Institut du Cerveau et de la Moelle Épinière and Pitié-Salpêtrière Hospital led by Prof. Lionel Naccache.
The researchers showed that the neurons in the visual cortex (in the vicinity of the Fusiform Face Area) responded much more strongly to faces than to city landscapes or objects. The brain’s response was much more energetic when the person saw famous actors or politicians than when looking at the Louvre in Paris, the Taj Mahal in Agra, India, a scissors or a backpack or, for that matter, faces unfamiliar to the participant in the experiment. In an additional experiment, the neurons exhibited face-selectivity to 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. In humans, face-selective activity has been extensively investigated, mainly using non-invasive tools such as functional magnetic resonance imaging (fMRI) and electroencephalograms,” explained Axelrod, the paper’s lead author.
“Strikingly, face-neurons in 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 largest and likely the most important face-selective region of the human brain.”
Probably the best-known neurons that respond to faces have been the so-called “Jennifer Aniston cells,” the neurons in the medial temporal lobe that respond to different images of a specific person (e.g., Jennifer Aniston in the original study published in Nature by Quiroga and colleagues in 2005). “But the neurons in the visual cortex that we reported here are very different from the neurons in the medial temporal lobe,” stresses Axelrod. “First, the neurons in the visual cortex respond vigorously to any type of face, regardless of the person’s identity. Second, they respond much earlier. Specifically, while in our case, a strong response could be observed within 150 milliseconds of showing the image, the “Jennifer Aniston cells” usually take 300 milliseconds or more to respond.”
The present results provide unique insights into human brain functioning at the cellular level during face processing. These findings also help bridge the understanding of face mechanisms across species (such as between monkeys and humans). “It is really exciting,” the Bar-Ilan researcher concluded, “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.”
