MORPHING mug shots of Margaret Thatcher and Marilyn Monroe together has helped to explain how the human brain recognises faces. And it seems separate brain regions handle the big picture and the details.
Combining the face of a 20th-century icon with the UK鈥檚 鈥淚ron Lady鈥� may strike some as sacrilege, but the technique allows researchers to distinguish between different aspects of recognition. For example, morphs can include different proportions of two unmorphed images, but still be recognisably of the same person.
Pia Rotshtein at University College London and her colleagues showed sequential, paired combinations of images of pairs of celebrities to 14 volunteers and measured their brains鈥� responses. Unknown to the volunteers, many of the images were morphs of the two people. 鈥淧eople will sometimes say, 鈥極h, I hadn鈥檛 realised that Maggie is so similar to Marilyn,鈥� but without realising it is an artificial face generated by morphing the two,鈥� says Rotshtein. 鈥淎 face that is 60 per cent Marilyn and 40 per cent Maggie will be identified as an older version of Marilyn,鈥� she adds, 鈥渨hile an image which is 60 per cent Margaret Thatcher will be seen as the sexier side of Maggie.鈥�
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The experiments used three kinds of image combinations. In one example, the first set was a pair of identical morphed images of 30 per cent Maggie and 70 per cent Marilyn. In the second set, that morphed image was paired with a 100 per cent photograph of Marilyn. The third set was two morphed images with compositions that differed by 30 per cent: a 70 per cent Marilyn put next to a 40 per cent Marilyn image.
Functional MRI of the volunteers鈥� brains showed which regions respond in each case (Nature Neuroscience, DOI: 10.1038/nn1370). The inferior occipital gyrus at the back of the brain was least active when images hadn鈥檛 been altered. But it did respond when the two pictures differed in the details, whether the identities of the women differed or not. The researchers believe this area registers physical details such as wrinkles, hairstyle, lighting and expression.
The right fusiform gyrus behind the right ear again did not respond when the pictures were identical, but nor was it active when two morphed pictures of the same woman were displayed. It showed most activity when the morph crossed the identity boundary though 鈥� for example, more Marilyn than Maggie. This area seems to register identity without any detail, filtering out everything not necessary for identification.
鈥淲ithout this ability we would not be able to recognise our friends in a dark pub and then on a bright sunny day, or when they change their hair style,鈥� says Rotshtein. Writing software to do this is fiendishly difficult, but our brains manage it in an instant, she adds.
