Star-gazers beware: Picking a celebrity’s face from a crowd might not be as easy as you thought – according to a new study by two York University professors.
"In the case of celebrities, many of whom have very distinctive facial features, our brains may not actually be able to tell the difference between a famous face and one that merely varies widely from the mean in the 
same general direction," says Hugh Wilson, a professor of biology in York’s Centre for Vision Research.
Left: From left, Hugh Wilson and Fran Wilkinson
The study is the first to present evidence that humans recognize faces based on distance from an average or "mean" face that is encoded by the brain. "You meet a lot of people and your brain stashes them all away, but the part of the brain that actually analyzes and classifies faces responds to geometric similarity," explains the study's co-author Fran Wilkinson, a professor in York’s Department of Psychology, Atkinson Faculty of Liberal & Professional Studies.
Right: Actress Gwyneth Paltrow has symmetrical features which vary slightly from the "average" female face
The pair used functional magnetic resonance imaging (fMRI) to study the changes in nerve cell activity that occur in an area of the brain called the Fusiform Face Area, which is known to be dedicated to visual perception of faces. They found that nerve cells in this area increase their firing rates as faces differ more and more from this average shape. Furthermore, different groups of cells respond best to different types of variation from the mean. For example, one group might respond best to faces that are much longer and thinner than the mean, with closely-spaced eyes and narrow mouths. Subjects were shown split-second images of human faces, some geometrically average, others differing to varying degrees from the mean.
Left: The average female face
How did they determine this average face? "We took a lot of digital photos," Wilson says. Each was mapped mathematically, allowing them to devise a geometrically typical face from the average of those values.
Aren’t faces that are beautiful closer to the average? "That’s an age-old debate," says Wilson, noting that research has shown faces that vary slightly from the mean to be universally more appealing to the eye.
For example, Gwyneth Paltrow's face is "almost perfectly symmetric," Wilson says, "which is a universal characteristic of mean faces that makes them appear beautiful. Contrasted with this, her wider mouth and eye separation give her a facial uniqueness and distinctiveness."
The paper, "fMRI evidence for the neural representation of faces," will be published in the October issue of nature neuroscience.