Prosopagnosia: the inability to recognize faces affects 3% of the population – Science et Avenir

A man shaves his hair in accordance with the rules and restores the shape. Outside, in the courtyard, he joins all his peers who have just arrived for military service. But he no longer recognizes any of them. In his eyes, suddenly all these men with the same hairstyles and outfits became clones. “Some people suffer from prosopagnosia without realizing it, they think that everyone is just like them,” comments Professor Olivier Martineau, a neurologist at the University Hospital of Caen. Prosopagnosia is the partial or complete inability to recognize a person by their faces. Rarely, though, it may affect more people than expected, suggests a new study published in the journal Cortex.

Prosopagnosia, or the inability to recognize a face

Humans are experts at face recognition. “For a social species like ours, faces are a valuable and diverse source of information,” explains Professor Laurent Cohen, a neurologist from La Salpêtrière (Paris). To recognize them, a certain area of ​​the brain is activated: the lower part of the temporal lobe of the right hemisphere. It is this area that is damaged in the so-called “acquired” prosopagnosia, especially after a stroke or accident, in contrast to the so-called “developing” prosopagnosia (from birth).

In the case of the latter, it is difficult to say where the dysfunction lies. “There are a lot of studies in monkeys that allow us to analyze in more detail what happens in these areas of the brain,” Laurent Cohen describes. “This shows that gradually, as information moves in the temporal lobe, different areas with different specializations extract the person’s personality.” The process of calculating personality from “increasingly abstract” information, in which each step can be affected by developmental prosopagnosia as the case may be. Brain systems that analyze the emotions of a face, the orientation of its gaze in space, or even lip reading, are not affected by prosopagnosia.

3% of the population will suffer from prosopagnosia.

Most prosopagnosias are developmental and affect one in 33 people in the United States, compared to one in 30,000 people with acquired prosopagnosia, explains Harvard neuroscientist Joseph DeGutis, first author of the new Cortex publication. In total, 2 to 2.5% of the population suffered from prosopagnosia. But in their study, the researchers test 3,116 Internet users and reach 3% of prosopagnostics using less stringent diagnostic criteria, but which they consider no less relevant. The level is “of the same order as most anomalies of cognitive development”, such as autism, dyscalculia or dyspraxia, comments Laurent Cohen. The authors report that in the study, prosopagnostics identified by strict tests showed statistically similar results to those identified by tests with milder thresholds. For them, this means that there is no real threshold at which it is possible to establish pathological functioning, and that prosopagnosia is rather a continuum.

“I don’t know if we can talk about a continuum,” comments Laurent Cohen. “Let’s say some people are better at recognizing faces than others, and prosopagnostics are at one end of that distribution.” The question is where to place the threshold. “In neuropsychology, if we take into account that the population is distributed along a Gaussian curve (Gaussian curve, editor’s note) for cognitive function, most people will have average values,” says Olivier Martineau. “The question then is to determine the threshold from which we speak of abnormal performance.”

However, prosopagnosia does not really follow this pattern. According to Olivier Martino, in terms of facial recognition, the majority of the population performs above average. From a research point of view, the article in Cortex is interesting, he continues. “Daily screening of people with lower academic achievement, even without disabilities, would allow larger groups of subjects to be included in studies and better inform the distribution of performance at the bottom of the Gaussian curve.”

Disability is preferred in the treatment of prosopagnosia.

Clinically, however, the question of the threshold that defines prosopagnosia is not very relevant, Laurent Cohen puts forward, especially since it is impossible to cure developmental prosopagnosia. “We offer rehabilitation, we help them implement alternative strategies to identify people without recognizing their faces,” Olivier Martino lists. Thus, he recalls the case of a prosopagnosic man who instructed his assistant to verbally name each person about to enter his office. In an interview with SoFoot, journalist Philippe Wandel recently acknowledged his prosopagnosia and explained how he distinguished football players despite their constant hairstyle changes. “I am more sensitive to physiognomy than to faces. For example, Kante, he has a lower center of gravity than others, he moves differently.”

While the discussion of the threshold and how to interpret diagnostic tests for prosopagnosia is always interesting, continues Olivier Martineau, in clinical practice the primary parameter remains the level of disability perceived by the patient. Thus, in developmental prosopagnostics, as in the person who discovers his disorder during military service, many moderate or mild cases live quite normally, often without realizing their peculiarity. “People can also be recognized by their voice, gestures, clothing, etc.,” explains Laurent Cohen. It tells the story of a boy who, on his way to a western movie, asks his father as he leaves his room why bandits cover their faces with a scarf. “It never occurred to him that a person can be recognized by his face!”

Secrets of the brain

While many people with developmental prosopagnosia probably do well with their disability, the disorder still holds many mysteries, clues to understanding how our brains work. “It has been shown that some prosopagnostics may also have difficulty not with reading, but with recognizing the handwriting style, for example, of a loved one!” says Laurent Cohen. Conversely, Alexis who can no longer read can still recognize the writing style. A feature that shows the complexity of the brain processes involved: “Does recognition of faces and writing styles depend on areas of the brain that are very close or have areas in common? We do not know”.

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