Although six-month-old babies are quite helpless in a lot of ways, it seems that they can outperform their older caretakers in at least one respect: distinguishing among faces of other species. Whereas adults can easily tell human faces apart but lack a similar ability to discern nonhuman primate visages, six-month-olds can recognize familiar faces of both humans and monkeys, according to a study published today in the journal Science.

Charles A. Nelson of the University of Minnesota hypothesized that as infants gain experience seeing faces, their brains tune in to the types of faces they see most often and become less receptive to those they see less regularly. If this is the case, the theory goes, younger infants¿who have had less time to become adept at discriminating among human faces¿should be better at differentiating among faces of other species. When Nelson and his colleagues Michelle de Hann of University College London and Olivier Pascalis of the University of Sheffield tested the reactions of six-month-olds, nine-month-olds and adults to color photographs of people and monkeys (see image), they observed exactly this. The researchers videotaped the subjects while showing them a unique human or primate face together with a previously viewed image. The team found that adults took longer to examine a novel human face compared with a previously viewed one but spent a similar amount of time examining both monkey pictures. The nine-month-olds demonstrated the same pattern. The six-month-olds, in contrast, spent significantly less time looking at pictures they had seen before than they did inspecting new photos for both species, which suggests they can discriminate between monkeys as well as humans. "We usually think about development as a process of gaining skills, so what is surprising about this case is that babies seem to be losing ability with age," de Hann says. "This is probably a reflection of the brain's 'tuning in' to the perceptual differences that are most important for telling human faces apart, and losing the ability to detect those differences that are not so useful."

Although it is still possible for humans to learn to recognize different faces of another species, the results indicate that the face-processing system for adults has a preferred human template. The fact that infants lose the ability to perceive facial differences, dubbed perceptual narrowing, may signal a general change in neural networks involved in early cognition, Nelson says. "We're interested in what this means in neurological terms," he adds. "For example, we don't know why this particular area of the brain¿the fusiform gyrus¿gets the assignment of distinguishing faces."

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