Trading faces DAVID ADAM If you never forget a face, you probably carry a ghostly, bland picture of Mr or Mrs Average in your mind's eye, new research suggests. This image helps you to recognize a long-lost friend with a big nose, for example, because their most striking feature is very different to the one on the face stored in your head. But live among big-nosed people for years, and your imaginary model could change — so that you unwittingly ignore your friend in the street.
Face-recognition theories agree that we have such a 'representative' face stored in our memories, against which incoming faces are checked. But the exact form of this visualized visage is unknown.
The new research, by David Leopold of the Max Planck Institute, Tübingen, Germany, and his colleagues, supports the idea that these images centre on a prototype face formed by averaging all the faces that that the brain has encountered.
But the research also suggests that this prototype is not fixed — rather, it can remould itself. And when the prototype changes, so too does the ability to identify familiar faces.
The results are "fascinating", says Anya Hurlbert, who studies face recognition at the University of Newcastle-upon-Tyne's Medical School. "It is entirely possible that the prototype may shift more permanently in response to more permanent changes in the faces we see — if, for example one moved from rural Iowa to a isolated village in mainland China," she says.
Leopold's team used the morphing technology beloved of filmmakers and advertisers to transform one face into another. They created a typical face, with features including nose size, distance between the eyes and forehead height averaged from 100 people.
They then chose four real people for comparison, but also generated an 'anti-face' for each. For instance, if Adam has a nose 50% more bulbous than average, then his anti-image was given a nose only half as thick as average, and so on.
The researchers also created weaker versions of both Adam and anti-Adam — effectively laying down a string of faces passing through the average. The team did the same for three other volunteers, Jim, John and Henry.
Next, they asked people to study the different versions of all four volunteers until they could name them even from the weaker, morphed versions. With practise they could identify Adam, for example, when faces with only one-third 'Adam-ness' flashed up for a split second.
But after gazing at the extreme 'anti-Adam' for five seconds before the tests, their perception changed. They could then recognize Adam from just one-tenth of his distinguishing features, but couldn't recognize weak pictures of Jim, John or Henry. The researchers report their results in the journal Nature Neuroscience1.
This effect quickly wears off but, over time, Leopold's group argues that it could change the brain's prototype face permanently. This would make certain faces harder to recognize because, for example, weak 'Jim' faces are more like Adam than the new anti-Adam prototype.
Still, the researchers' theory is not the only way to explain their results, Hurlbert says. The after-effects could be due to changes in sensitivity to isolated features such as the eyes rather than wholesale facial changes.
Leopold, D. A., O'Toole, A. J., Vetter, T. & Blanz, V. Prototype-referenced shape encoding revealed by high-level aftereffects. Nature Neuroscience 4, 89–94 (2001).
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