The researchers, led by Ken Nakayama and Richard Russell at Harvard and Bradley Duchaine at University College London, have found evidence that prosopagnosia, once thought to be exceedingly rare, may affect up to 2 percent of the population -- suggesting that millions of people may be face-blind.
"Until a few years ago, only 100 cases of prosopagnosia had been documented worldwide, but it now appears the condition is much less rare than had previously been assumed," says Nakayama, the Edgar Pierce Professor of Psychology in Harvard's Faculty of Arts and Sciences. "Testing of 1,600 individuals found that 2 percent of the general public may have face-blindness and a German group has recently made a similar estimate. It's conceivable that millions of people may have symptoms consistent with prosopagnosia, without even realizing it."
Tests developed by Nakayama and colleagues display dozens of images of cars, tools, guns, houses, and landscapes, along with close-cropped black-and-white pictures of faces. Some of the images recur during the cycle; subjects are asked to indicate, as quickly as possible, whether each image they see is new or repeated. Prosopagnosics who take these tests fail to recognize repetition among the faces in the series, even as they readily identify repeated pictures of other objects.
Reports of prosopagnosia date back to antiquity, although it wasn't until 1947 that researchers made the first modern, extensive description of symptoms. The neurological basis of the disorder remains poorly understood, although research has c onfirmed that the brain processes faces differently than other kinds of objects.
The condition can be embarrassing and lead to social isolation: Severe prosopagnosics may mistake complete strangers for acquaintances even as they fail to recognize family members, close friends, spouses, and even themselves. Many report difficulty watching television shows and movies, since they cannot keep track of characters. Face-blind individuals often compensate for their prosopagnosia using non-facial traits, such as hair, gait, clothing, voice, and context.
There are two broad categories of prosopagnosia. Most documented cases have been of acquired prosopagnosia, due to brain damage suffered after maturity from head trauma, stroke, and degenerative diseases. The other type is called developmental prosopagnosia. These individuals can have similar symptoms but with no evidence of brain damage. It is this group that may be the most common.
"Individuals who've been face-blind since childhood often do not realize that they are unable to recognize faces as well as others; they have never recognized faces normally so their impairment is not apparent to them," says Russell, a postdoctoral researcher in Harvard's Department of Psychology. "As a result, many individuals do not recognize their prosopagnosia until adolescence or adulthood."