Sure faces are special, but why?

Faces are special. There appears to be a dedicated area of the brain for processing faces. Neonates just a day or two old prefer looking at pictures of faces to looking at non-faces.

This has led many researchers to claim humans are born with innate knowledge about faces. Others, however, have claimed that these data are not the result of nature so much as nurture. Pawan Sinha at MIT attached a video camera to his infant child and let the tape roll for a few hours. He found that faces were frequently the most salient objects in the baby's visual field, and (I'm working from memory of a talk here) also found that a computational algorithm could fairly easily learn to recognize faces. Similarly, a number of researchers have claimed that the brain area thought to be specialized for face detection is in fact simply involved in detecting any object for which one has expertise, and all humans are simply face experts.

Things have seemed to be at an impass, but today, Yoichi Sugita from AIST spoke at both Harvard and MIT. The abstract itself was enough to catch everybody's attention:

Infant monkeys were reared with no exposure to any faces for 12 months. Before being allowed to see a face, the monkeys showed preference for human- and monkey faces in photographs. They still preferred faces even when presented in reversed contrast. But, they did not show preference for faces presented in upside-down. After the deprivation period, the monkeys were exposed first to human faces for a week. Soon after, their preference changed drastically. They preferred upright human faces but lost preference for monkey faces. Furthermore, they lost preference for human faces presented in reversed contrast. These results indicate that the interrelated features of the face can be detected without experience, and that a face prototype develops abruptly when flesh faces are shown.

Just to parse this: the monkeys were raised individually without contact with other monkeys. They did have contact with a human caregiver who wore a mask that obstructed view of the face. The point about not preferring upside down faces is important, as this is a basic feature of face processing.

This seems pretty decisive evidence for an innate face module in the brain, though one that requires some tuning (the monkeys' face preferences evolved with experience). However, Sugita apparently noted during the talk -- I heard this second-hand -- that perhaps the monkeys in question did in fact have some experience with faces prior to the face preference test; they could have learned by touching their own faces. This strikes me as a stretch, since that doesn't explain why they would become face experts.