As a child, Heather Sellers sensed something wasn't normal, but she had no idea what it was. "I would be talking to someone who clearly knew who I was, but I didn't know who they were," she recalls.

She didn't recognize the faces of classmates, and she had difficulty making and keeping friends. Even faces of her own family were unfamiliar. Sometimes in the pickup line after school, Sellers would walk right past her mom if her mother wasn't wearing the same clothes she'd had on in the morning.

It wasn't until Sellers was in her thirties that she came across the concept of "face blindness." She instantly saw herself in the description and was inspired to learn more.

"I had never heard of the condition before," says Sellers, who teaches writing at the University of South Florida in Tampa. She eventually underwent evaluation and testing for the disorder with researchers at Harvard University and MIT in Cambridge, MA.

Face blindness, officially called prosopagnosia, is a neurologic disorder in which a person's ability to recognize and identify faces, sometimes even his or her own, is impaired—and not because of poor eyesight or absentmindedness, says Brad Duchaine, PhD, professor of psychological and brain sciences at Dartmouth College in Hanover, NH. Rather, prosopagnosia involves irregularities in the neural network involved in recognizing and identifying faces, he says. The condition is one of several disorders categorized as visual agnosia, which affects the brain's processing of incoming visual information.

It is estimated that 2 percent of the population has prosopagnosia to some degree, according to a study by Dr. Duchaine that was published in Current Opinion in Neurobiology in 2013. Most have the developmental form, meaning they were born with the condition, and it often runs in families, so genetic factors may play a role, says Dr. Duchaine. It also sometimes manifests with other disorders, such as autism. Less commonly, acquired prosopagnosia may result from brain damage due to stroke, head trauma, or neurodegenerative disease, adds Dr. Duchaine.

To compensate for not recognizing faces, people learn to use cues like hairstyle, gait, or voice to identify others, he says. In rare instances, people can't distinguish faces from objects; in his book The Man Who Mistook His Wife for a Hat (Summit Books, 1985), the late neurologist and author Dr. Oliver Sacks chronicled one such case. Dr. Sacks himself experienced face blindness. He also had trouble recognizing places.

Not until the past few decades have scientists realized how many people have problems recognizing faces, says Dr. Duchaine. New technologies have helped diagnose them, including brain imaging techniques such as MRI and functional MRI that show areas of the brain lit up during a given function. Electroencephalography (EEG), which allows neurologists to study patterns of electrical signaling in the brain, also provides insight.

Cases of acquired prosopagnosia are aiding scientists in pinpointing the regions of the brain crucial to perceiving and identifying faces, says Steven Galetta, MD, FAAN, professor and chair of neurology at NYU Langone Medical Center, who has treated such patients.

Network Snag

The brain's multistep process for interpreting visual information is complex, Dr. Duchaine says. It's likely there are different types of prosopagnosia, each one caused by a different glitch in that network. "Evidence from a variety of sources demonstrates the right temporal lobe is critical for processing images of faces," he says, and a specific region called the fusiform gyrus seems particularly important.

A range of ability exists when it comes to recognizing faces, says Joseph DeGutis, PhD, neuroscientist and assistant professor of psychiatry at Harvard Medical School. On one end of the spectrum are people with prosopagnosia who can't recognize faces at all. In the middle are people who have a so-so ability. "At the other end of the spectrum are 'super recognizers,'" Dr. DeGutis says. "Not only do they recognize a person they saw once on the train, but they can remember contextual details, such as 'I sat next to you on the train three months ago, and you were wearing a red jacket.'"

Face perception involves integrating information about multiple body parts—eyes, ears, nose, mouth—into a single whole, says Dr. DeGutis. Perceiving the distance between the eyes and eyebrows and the distance between the mouth and nose are important to recognition. Some researchers believe "you read a face like a vertical bar code," from top to bottom, he says.

Learning Tools

To help people with prosopagnosia learn to perceive faces better, Dr. DeGutis and his team have developed a web-based program with funding from the National Eye Institute. Participants are taught to look for different configurations. For instance, one face might have higher eyebrows and a lower mouth while another has medium eyebrows and a higher mouth. It's challenging at first, but over time participants can perceive these subtle differences more accurately and more quickly, says Dr. DeGutis, who co-authored a paper on the program in the journal Brain in 2014.

The program hasn't helped everyone, but in one group that underwent training, 50 percent noticed positive changes in their everyday life, he says. One woman who did several months of training reported "significant improvement in daily life," Dr. DeGutis says. "She was recognizing familiar faces out of context, like her neighbors at the grocery store."

People with prosopagnosia do not see a blur when they view other people's faces, explains Dr. Duchaine. Instead, they see upright faces only as clearly as other people see upside-down faces. The deficit can be profound, says Dr. Galetta. "Recognizing people's faces is so important in social interactions. To lose that ability makes that interaction awkward." He recalls one patient who developed the condition following a stroke. The man worked in a tollbooth and enjoyed greeting familiar faces who passed through, a satisfying aspect of his job he was sad to lose.

Another patient developed prosopagnosia after a brain vessel ruptured due to amyloid buildup and caused some brain damage. She couldn't identify her closest friend until she heard her voice, and for a while didn't recognize herself in the mirror.

"You know somebody for a long time, and now you don't recognize him or her. You feel bad, and the person you're talking to may feel bad too," Dr. Galetta says. "Prosopagnosia can change your whole social network and the things you value."

Dr. Duchaine maintains a website called faceblind.org that provides a five-minute face-recognition test and allows people to indicate whether they'd like to be contacted to participate in research. Heather Sellers used this site, and later wrote about her prosopagnosia in a book called You Don't Look Like Anyone I Know (Riverhead Books, 2010).

Before she was diagnosed with prosopagnosia, Sellers found ways to compensate. When she was in school, she recognized fellow students by where they sat in class and whether they were funny or outgoing. A child with red hair was easy to spot. She became friends with a boy who used a wheelchair, probably in part because she could perceive the chair.

Sellers said that finally getting a diagnosis of prosopagnosia was life-altering. Instead of hiding the fact that she couldn't recognize faces, she started letting people know. In her college classroom, she asks students to wear name tags, which has the added benefit of helping them learn other students' names.

By telling people about her condition and asking them who they are, she finds out more about them. "What I like about my condition is that it forces me to ask my fellow human beings for help," says Sellers. "It helps me to connect."