35 years after leaving school, the majority of people can still identify 90% of their classmates.
You'd know that face anywhere? Then thank your right fusiform gyrus. Scientists have identified the bits of the brain that can tell Tony Blair from James Bond, or whether Lady Thatcher has borrowed Marilyn Monroe's hairstyle.
Pia Rotshtein of the institute of neurology at University College London and colleagues used sophisticated scanning equipment to monitor the brains of volunteers while they watched Marilyn Monroe morph into Margaret Thatcher and the current prime minister turn into Pierce Brosnan.
The experiment, reported in the online edition of Nature Neuroscience today, pinpoints the three parts of the brain that light up in face recognition. One bit studies the physical aspects, one identifies the face as known or unknown, and the third retrieves the name or other facts linked to that face. The study helps explain that nagging feeling that you know a face but cannot place it.
"Recognising people is an essential skill we often take for granted," Ms. Rotshtein said. "Most of us can recognize someone even if we haven't bumped into them for 10 years."
Humans can remember up to 10,000 faces. In experiments, 35 years after leaving school, people have proved able to identify 90% of their classmates.
Using functional magnetic resonance imaging, the scientists found that the anterior temporal cortex became more active when the volunteers knew their celebrity well but were hardly active at all in a Polish volunteer who was shown a photograph of John Major, the prime minister from 1990 until 1997.
The right fusiform gyrus, located just behind the ears, lit up when volunteers looked at a somewhat changed face and compared what they saw to stored memories. And the inferior occipital gyri located at the back of the brain were sensitive to slight physical changes in the morphed faces.
Ms. Rotshtein worked with morphed studies of familiar faces. The challenge was to see how the brain matched an identity to a significantly changed face.
"Our study shows the brain tries to force us to pin a single identity to a face. So a face that is 60% Marilyn Monroe but 40% Margaret Thatcher will be identified as an older version of Monroe, while an image 40% Monroe and 60% Thatcher will be seen as the sexier side of Thatcher."
Damage to any of the three parts of the brain precipitates a crisis of recognition. Dementia patients with damage to the anterior temporal cortex have a problem finding the name to go with the face, while people with epilepsy triggered by the right fusiform gyrus sometimes believe different faces belong to the same person, says Jon Driver of the institute of cognitive neuroscience at UCL, one of the research team.