RENÉ MAGRITTE, a surrealist artist, perfectly captured the idea of prosopagnosia, or face-blindness, in his painting “The Son of Man”. In the picture, an apple floats in front of a man's face, covering the features that would normally allow him to be recognised.
To people with prosopagnosia, the instant someone leaves their sight the memory of that person's face is blank—or, at best, a palette of muddled features. Face-blindness can be likened to tone-deafness: the tone can be heard, or the face seen, but distinguishing between different tones or faces is nearly impossible. The effects of prosopagnosia can be so bad that people with severe cases cannot recognise their own parents or children.
Understanding the face-recall mechanism in the brain would be a big step towards understanding this odd disorder. It might also throw light on human evolution, since the ability to recognise faces is more or less equal to the ability to recognise individuals, and that ability is the glue which holds societies together and which has enabled humanity to develop a complex culture unique in the animal kingdom. The question is whether this crucial ability has its own private brain mechanism, or whether it is simply one aspect of a general ability to recognise individual members of a particular class of objects.
A paper by Brad Duchaine and Ken Nakayama of Harvard University, which is to be published shortly in the Journal of Cognitive Neuroscience, uses face-blind volunteers to explore this question. In the study, Dr Duchaine and Dr Nakayama showed their seven face-blind subjects a series of images of cars, tools, guns, houses and landscapes, and also black-and-white pictures of faces without hair on their heads (prosopagnosiacs tend to use cues other than facial features to distinguish between people). Ten of these images were repeated during the cycle. The subjects were asked to indicate, as quickly as possible, whether each image they saw was new or repeated.
If, the researchers reasoned, visual recognition is done by one type of brain process regardless of what is being recognised, then the face-blind subjects would show difficulty in recognising all repeated objects in the series, not just the faces. They found, however, that while none of their prosopagnosiac subjects could recognise the faces in the series well, they could distinguish between the other repeated pictures as easily as people without prosopagnosia could. That confirms the idea that faces are handled differently in the brain from other objects.
Further evidence supporting this idea comes from a study that Dr Duchaine and Dr Nakayama published earlier this year in Neuron. This reported the experience of a physicist known as Edward, who is severely face-blind. It was designed to test the idea that face recognition is a type of expertise that is learnt, in the same way that judges at dog shows, or connoisseurs of antiques, are trained to notice fine visual distinctions in their fields.
One way psychologists study the acquisition of such expertise is to use computer-generated images of objects known in the field as “greebles”. A greeble looks like a space alien. It has features that might be interpreted as limbs, sense organs and the like, and these features can be manipulated so that greeble families—of similar but not quite identical individual greebles—can be generated. Greeble training consists of recognising both the differences and the similarities.
As it turned out, Edward did well in greeble training, easily mastering the finer points of greeble anatomy. His ability to become an expert at appraising these objects showed that prosopagnosia is not caused by an inability to make fine distinctions between objects that are basically similar. Face-recognition, it suggests, is a specific skill that Edward simply lacks.
A third study, published in this week's Neuron by Galit Yovel and Nancy Kanwisher of the Massachusetts Institute of Technology, digs a little deeper into the question. Dr Yovel and Dr Kanwisher have been studying what happens in people's brains when they look at faces.
To do this, they used a brain-scanning technique called functional magnetic-resonance imaging, which follows the brain's activity in real time. Their area of particular interest was part of the brain called the fusiform face area (FFA), which previous work had suggested is involved in the recognition of objects, including faces.
Dr Yovel and Dr Kanwisher showed their subjects pictures of faces and houses, and measured the level of activity in the FFA. They found that it was far more active when someone was looking at a face rather than a house. They also found that the pattern of activity changed as they changed the pattern of features on a face. The FFA was the only part of the brain to respond this way, so it looks as though Dr Yovel and Dr Kanwisher have found the location of the face-processing mechanism. The next step will be to look at what happens there when prosopagnosiacs are presented with faces.
So, while science has yet to pin down what it is that people with prosopagnosia are missing when they recall a blur instead of a face—and therefore what allows everybody else to be so good at recognising one another—it is making good progress towards that goal. This is not to say that prosopagnosia has no advantages. As one person with it posts on her website, “You can wake up in the morning and pretend you don't know your kids. Then you don't have to pay them an allowance.”
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