Researchers recognize two types of prosopagnosia—a rare, acquired type, and a more common form called developmental prosopagnosia. People with the acquired type have lost the ability to recognize faces due to some sort of brain injury, such as a stroke. For those with the other type, certain brain mechanisms failed to develop properly, perhaps for genetic reasons (it does seem to run in families). While cases of acquired prosopagnosia have been known since the mid-19th century, the first report of developmental prosopagnosia didn’t appear until 1976. “It took us a lot longer to recognize it,” says Brad Duchaine, an associate professor in the Department of Psychological and Brain Sciences at Dartmouth College who has been studying prosopagnosia for years. “You can imagine if you’re an acquired prosopagnosic, well, one day you could recognize people and the next day you couldn’t. So it’s much more apparent to people.” To perceive and recognize a face, the brain relies on a neural network of at least three core regions that seem to contribute to different aspects of face processing. There’s debate over whether or not this network is specialized for processing faces alone, or if it’s also used to discriminate among other visual objects associated with expertise, such as birds for birders. These regions are found in the occipital and temporal lobes of both the right and left hemispheres, although the right side seems to be more active in face processing, according to neurologist Jason Barton, who runs the Human Vision and Eye Movement Laboratory at the University of British Columbia. Damage to different parts of this neural network can interrupt different aspects of the face-recognition process. For instance, work by Barton and others have shown that lesions in certain regions in the right occipital lobe can inhibit people’s ability to perceive faces—that is, faces just don’t register. “When they look at a face, they can’t see enough of the details in the face to know who that is,” says Barton. “It’s as if they’re looking at a face through a fog.” It’s less clear what causes face-blindness in people with developmental prosopagnosia. Neuroimaging studies have suggested that there are structural and functional anomalies in the brain’s wiring, according to Barton, but there’s no consensus on a defining abnormality or genetic marker. For this reason, the line between having developmental prosopagnosia and being simply “bad” with faces can be blurry, according to Barton. “One of the things about any kind of human ability is that we’re not all the same,” he says. “There is a distribution of ability.”Prosopagnosia varies in severity, depending on the individual—that is, different people can have different degrees of difficulty recognizing and recalling faces. In a 2010 article for The New Yorker, the late neurologist Oliver Sacks—who said he had the developmental kind—wrote that he and others with “moderate prosopagnosia,” can, “after repeated exposure, learn to identify those they know best.”
Story by Jolene Manna.