Imagine scanning a crowd, looking for a familiar face, but everyone has the face of a stranger. That's what happens every time Heather Sellers walks into a restaurant.
"I am known to everybody else, and I don't know if I know them or not," Sellers said.
Some people never forget a face. But for 40 years, Sellers, a college English professor, has never been able to remember one. Even a face she's known since birth.
"I wouldn't be able to recognize my mother out of context if she was walking down the street. And then, along with that, I mistake people for her," Sellers said.
Growing up, Sellers had difficulty making friends. Later in life, not being able to recognize her co-workers, she said, strained relationships and hurt her career.
"I avoided a lot of committee meetings and a lot of my duties at my college, because it was so confusing to go into a room after five years and still not know who these people are," Sellers said.
She avoided parties and social gatherings altogether, because they were torture for her. Amid a sea of faces, she'd have trouble picking out even her closest loved ones.
"I could not recognize my husband, my then-husband, in the grocery store [or] in my own backyard," Sellers said.
Sellers suffers from a neurological disorder called prosopagnosia, or face blindness. The instant someone leaves her sight, the image of that person's face fades from her memory.
It's not that Sellers has a bad memory -- she can recall names, phone numbers, even a book she's read with ease. But mysteriously, she is unable to recall faces. Even her own image in a mirror throws her off.
"There is just a sea of faces that are reflected," Sellers said. "I won't know which one is me. It's frightening, and it's confusing. And I didn't want anyone to know. I thought I was crazy. "
Sellers isn't alone. Jim Heard, a retired art teacher, has spent his entire life pretending to recognize faces when often he can't.
"You hide it. ... You think you're the only person with the problem," said Heard.
"I recognize people -- it's just faces I don't recognize. So I will recognize voices, I will recognize the way somebody walks, all those kinds of little things that you put together that makes a person an individual."
Among the faces Heard struggles to recall are those in his own family. But that problem is shared by other family members. Two of his daughters -- Catherine, an artist, and Jayne, a neuroscientist and mother of two small children -- have trouble recalling his face, too, as well as their mother's.
"It was always common family knowledge that Jayne and my father and I were very bad at recognizing people," Catherine said.
"We'd meet somebody in public, and Mom would have to tell Dad, 'Oh, I think that's one of your students, Jim,'" Jayne added.
Heard said he just thought he was an absent-minded professor. "I didn't have a clue as to why I was like that."
It's not that prosopagnosics have difficulty with vision. They can see faces perfectly well. They just don't know whose face they are looking at, and they won't remember it once they look away.
Nancy Kanwisher investigates visual perception and cognition, and runs a brain research lab at MIT.
"Prosopagnosia subjects can see the face. They know it's a face. They just don't know which face it is. They don't know who it is," Kanwisher said.