Imagine walking into the coffee shop where you've been meeting your best friend every day for a year and glazing over her face as if she were a stranger, unable to pick her profile out of the crowd.
Now imagine getting on a crowded train and spotting the person who bumped into you at the grocery store six years ago.
Each of these situations is considered a normal occurrence for roughly 1% of the population. The first scenario might happen frequently to a "prosopagnosic," someone who is face blind. The second is a typical experience for a "super-recogniser," an individual who scientists now believe has the uncanny ability to place a familiar face — even that of a practical stranger.
Psychologists, neuroscientists, and even members of the police want to better understand the human brain's remarkable capacity to recognise and identify faces — and they're starting to do so by studying these two groups.
There's something special about faces
Research suggests that facial recognition is fundamentally different from traditional memory in several key ways. First, you can't learn it. Second, it has a structural signature in the brain which scientists think they might soon be able to measure.
Compared with a face-blind person, for example, a super-recogniser might have a slightly larger fusiform face area (a special region of the brain that's thought to play a key role in our ability to recognise faces). Or, a super-recogniser might show more activity in this area when she is shown pictures of faces.
"Any time there's a psychological difference there has to be a neurological basis," Brad Duchaine, a neurologist at the University College London, told Business Insider. "Just like you'd say, okay, that car is faster than that other car. Is there a difference in their engines? Well yes of course there is."
At present, Duchaine and other researchers lack the data to confirm this. Most existing studies of super-recognisers and prosopagnosics are relatively small; many of the most intriguing hypotheses about both groups' abilities have not yet been published in peer-reviewed journals.
Nevertheless, some people are putting the ideas into practice. The London police, for example, have a special task force called the Super Recogniser Unit. Officers in the unit, who are believed to be able to accurately identify people from grainy, poor-quality surveillance footage, are being called in to help crack cases that have gone cold.
Eliot Porritt, who leads the task force and serves as a detective sergeant with the London Metropolitan Police, told Business Insider that since the program launched in 2011, it's been hugely successful in helping to identify suspects.
"It's all very well saying you know I think I've seen that guy before. But where and who is he? That's always the question that my unit tends to ask," said Porritt.