A surprising number of people can't recognize faces - sometimes even their own

• Some people have a condition that makes it difficult for them to recognize familiar faces, even those of friends and family. Sometimes they may even have issues recognizing themselves. This is called prosopagnosia.
• Prosopagnosia is estimated to affect about 1% of the population.
• It's not necessarily linked to brain damage or any other intellectual problem; the prolific author and neurologist Oliver Sacks was a prosopagnosic.
• Merriam-Webster added prosopagnosia to the dictionary earlier this week.

I've seen a lot of faces that I can't forget.

No, I'm not talking about being in love, as The Beatles lyrics might imply. Instead, I may be a super-recognizer, meaning that other peoples' faces get strangely seared into my brain - even those of complete strangers. It's not that I necessarily want to remember them - I just can't seem to help it.

And I'm not alone. Josh P. Davis, a psychology professor at the University of Greenwich in England who studies the phenomenon, told me he estimates some 1% of the population could qualify as super-recognizers. (Keep in mind, Davis said - "if you do very well then you may be a super-recognizer" - if you want to know for sure, you can inquire with his team about additional testing.)

But it turns out that another 1% of the population appears to fall into a category that might be best-described as the opposite of super-recognition.

These face-blind people, or "prosopagnosics," a term that neuroscientists have used to refer to them for several years but was only officially added to the Merriam-Webster dictionary this month, essentially cannot recognize familiar faces.

'Prosopagnosia' is essentially face blindness

The word prosopagnosia, which combines the Greek words "prosopon," or face, with "agnos," or lack of knowledge, dates back to when researchers first identified the "condition" in people with brain damage to a specific area of the brain called the fusiform face area, or FFA, about 30 years ago. Scientists believe the FFA is thought to play a key role in our ability to identify a face.

Prosopagnosics, they found, had severe difficulties recognizing familiar faces - even, sometimes, their own.

But you don't have to have experienced brain damage to have prosopagnosia.

More recently, researchers have diagnosed the condition in perfectly healthy people who appear to be born with it. The deficit, called "developmental prosopagnosia," doesn't appear to negatively affect other intellectual efforts in these people.

Oliver Sacks, the renowned neurologist and prolific writer, for example, was a prosopagnosic. He wrote extensively about his condition in the book "The Mind's Eye."

"I am much better at recognizing my neighbors' dogs (they have characteristic shapes and colors) than my neighbors themselves," Sacks wrote.

'Super-recognition,' on the other hand, is a superior ability to pick out a familiar face

In the first paper to mention the phrase "super-recognizer," published in 2009, University College London cognitive neuroscientist Brad Duchaine, along with Harvard psychologists Ken Nakayama and Richard Russell, outlined the experiences of four people who claimed to have an unusually good ability to recognize faces. In addition, the researchers presented the world's first test designed to identify these so-called super-recognizers, the Cambridge Face Memory Test, which you can take online now.

All four subjects in the paper described eerie instances in their past in which they had recognized apparent strangers: family members they hadn't seen for decades or actors they'd glimpsed once in an ad and then seen again in a movie. Each person in the study said that for years they'd felt as if something were wrong with them. One of the participants, for example, told the researchers she tried to hide her ability and "pretend that I don't remember ... because it seems like I stalk them, or that they mean more to me than they do."

For the first time, the Cambridge test suggested to these people that they weren't alone - that their abilities weren't merely in their head but quantifiable, testable, able to be proved and put down on paper.

What we know - and don't know - about facial recognition

Research suggests that facial super-recognition is fundamentally different from traditional memory in several key ways. First, the ability doesn't appear to be able to be learned or enhanced with training. Second, it appears to have a neurological and structural basis.

But there's still a lot we don't know about super-recognition - and about facial recognition more broadly.

In a recent study in the journal PLoS One, for example, researchers studied two so-called memory champions, people who had competed extensively in memory contests and had even been recognized by the Guinness World Book of Records for their memorization skills. When the researchers studied these people's facial-recognition abilities, however, their results were merely average. In other words, the researchers concluded, something about facial processing was fundamentally different from memory - and it couldn't be learned by any training or class. Instead, it seemed to be innate.

And if people are born with their facial-recognition abilities, then they most likely have a neurological basis in the brain, researchers say. A super-recognizer, for example, might have a slightly larger fusiform face area than a face-blind person, or the person might show more activity in this area when looking at images of a face. "Any time there's a psychological difference there has to be a neurological basis," said Duchaine, the University College London cognitive neuroscientist. "Just like you'd say, OK, that car is faster than that other car. Is there a difference in their engines? Well yes of course there is."

Still, Duchaine and other researchers lack the data to confirm this. All of the existing studies of super-recognizers are based on very small samples of people - anywhere from just two individuals to a half-dozen people. Several of the researchers have presented their hypotheses about super-recognizers at conferences and presentations, but many of these haven't yet been published in peer-reviewed journals.

So what exactly are prosopagnosia and super-recognition? Skills? Signs of intelligence? Quirky developmental traits?

That's something that remains to be seen.