How does a parent sort one identical twin from another? Why can you recognise former school friends decades after graduation, even if you haven't seen them since? How can you keep names attached to the hundreds of people you meet in the course of a given year?
The brain has evolved very elaborate methods for sorting one face from another but there are differing views of how this is achieved. Researchers at Max Planck's Institut fur Biologische Kybernetik believe the brain creates a face "prototype" and then compares real faces to this averaged version.
Dr David A Leopold and colleagues describe their findings in the January issue of Nature Neuroscience. They were trying to understand how representations of familiar faces are stored in memory.
Faces they say are largely similar and differ only in subtle aspects of "shape, texture and colour". Recognition, they say engages specialised neural mechanisms and where these are in the brain have been identified in both monkeys and humans. Yet this knowledge does not tell us how these mechanisms contribute to recognition.
The authors focused on a phenomenon called visual "after effects". These are distortions to how we perceive things brought about intentionally by controlling what the eye is allowed to see.
In this case subjects were asked to look for several minutes at distorted faces, for example grotesquely compressed or expanded faces. For some minutes afterwards, the subjects' ability to recognise normal faces was changed, a process called adaption.
Adaption is simple to demonstrate. If you stare at a red square for a minute or two and then look at a white sheet of paper, you will see a green square, a colour opposite to the original red.
Adaption also takes place when looking at faces. After seeing the intentionally distorted faces the subjects adapted and for a time afterwards normal faces seemed distorted in the opposite way. The effect is real but short-lived and within minutes it fades.
"This effect depended on the nature of the gross distortion, and was non-specific with regard to the identities of the faces," the authors say in Nature Neuroscience.
To understand what was happening, they created a library of faces that differed in systematic ways from one another. They showed they could cause a subject to adapt when looking at faces and could also control the nature of the adaption. It was so effective that the subject often couldn't recognise very familiar faces until the adaption disappeared.
The researchers' explanation for what was happening in the brain is intriguing. They argue that we create a prototype face, one formed by averaging together all the faces the brain has ever seen. We compare the faces we see each day against the averaged face and then store the differences to help us remember and recognise people.
If we view distorted faces our prototype changes under the influence of adaption and so our stored memory of familiar faces doesn't work.
While the work might seem obscure it provides important insights into how our minds process information. Face recognition is a vital process and would have exerted a strong evolutionary pressure as the human brain evolved.