PRESENTER: This week's "All in the Mind" examines an unusual condition you may never have heard of before: prosopagnosia. Here's Professor Alexander Scharma to explain. PROF. SCHARMA: Hello. Well, let's start with an image some of you may be familiar with: a painting called "The Son of Man", by the surrealist artist René Magritte. In the picture, an apple floats in front of a man's face, covering the features that would normally allow him to be recognised. The painting perfectly illustrates the concept of prosopagnosia, or face-blindness. To people with this condition, as soon as someone leaves their sight the memory of that person's face is blank - or, at best, a set of jumbled features. Face -blindness is a little like 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 severely affected cannot recognise their own parents or children. If we understood how the normal brain recalls faces, we would be well on the way to understanding this strange disorder. It might also help us to understand human evolution, since the ability to recognise faces is more or less equal to the ability to recognise individuals. This ability helps to hold societies together and has enabled humanity to develop a complex culture which is unique in the animal kingdom. The question scientists need to answer is whether this basic 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. Researchers have used face-blind volunteers to explore this question. The subjects were shown images of cars, tools, guns, houses and landscapes, and also black-and-white pictures of faces with no hair on their heads. Ten of these images were repeated. The subjects were asked to indicate, as quickly as possible, whether each image they saw was new or repeated. The results were surprising. None of the face-blind subjects could recognise the faces in the series well, but they could distinguish between the other repeated pictures as easily as people without prosopagnosia could. That confirms the idea that faces are handled differently by the brain from other objects. It has been shown in experiments that people with face-blindness can be taught to improve their face recognition skills, but it is still not known what prosopagnosia sufferers are missing when they recall a blur instead of a face. This is not to say that prosopagnosia has no advantages. As one person with the condition writes on her website, 'You can wake up in the morning and pretend you don't know your own kids. Then you don't have to give them any pocket money'.