Searching for a friend's face in a sports arena packed with spectators can be difficult, but not impossible. But what if all the seats were filled with macaque monkeys--could you still find that one-in-a-thousand face? A six-month-old baby might fare better at this task than an adult, according to a study in the 17 May 2002 Science, published by the American Association for the Advancement of Science.
Three scientists set out to determine whether face recognition abilities follow a similar fate as one's ability to learn new languages--which gets worse with age. According to University of Sheffield's Olivier Pascalis and colleagues, people tune their brains to the faces they see the most within the first year of life, hard-wiring a template against which to compare new visages. They discovered that six-month old infants easily distinguish between individual humans and individual monkeys. Babies older by only three months and adults have an easy time telling apart fellow humans, but telling apart one monkey from another proved nearly impossible.
"There are these really remarkable changes in the first year of life in the face processing system," said Michelle de Haan, a co-author at Institute of Child Health, University College London. "Face recognition is one example of the way we study how the brain is divided up to different areas that serve different functions. 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. 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."
This evidence for "perceptual narrowing," as also displayed when we lose abilities to differentiate between non-native sounds, such as those from foreign languages--and not because of memory deterioration--may represent a more general change in early neural network development than previously thought, suggest the researchers.
"It's one way of looking at the bigger question of how brains develop into the adult like form: When babies are born, do they already have this little face and language area, do they have areas dedicated to specific functions?" explained de Haan. It is possible that a combination of natural changes in the brain between six to nine months of age and visual cues from the environment hone recognition capabilities, she said.
Brain systems for processing visual cues and speech may develop with a similar timing and influence each other and provides ample areas for further studies, the researchers write. "Although it is difficult to compare directly the tuning of speech perception with the tuning of face perception, there may be overlap between these systems."
By adulthood, individual faces are "encoded" in terms of how they deviate from the brain's standardized face. The researchers knew from previous studies that adult humans are better at telling apart human faces than monkey faces, and monkeys, in turn, are better at telling apart other monkeys. Whether this was the case for infants with little experience with either species inspired the researchers to conduct several tests.
Two initial tests set the premise for the study. The researchers first observed brain waves to confirm young infants, but not adults, could discriminate monkey face identity across changes in facial orientation. In a second behavioral test turning the faces upside down only affected adults' brain waves for human faces, whereas in infants, it affected their brain waves similarly when identifying both humans and monkeys. These results provided some of the first clues that young infants might be better than adults at recognizing monkey faces.
The major experiment, called a "visual paired comparison procedure," assessed recognition abilities in both babies and adults. Eleven adults and 30 healthy, full term six-month-olds and 30 healthy, full term nine-month-olds were shown pairs of colored pictures of male and female Caucasian faces and monkey faces, the latter of Macaca fascicularis species. Of the two pictures, one was never shown before, and another was familiar from a prior viewing. The researchers detected recognition by videotaping and recording the participant's tendency to stare at the new item longer.
Adults took longer to examine new pictures of human faces compared to older faces, but they looked just as long at the new and old monkey faces. Similarly, nine-month-olds looked longer toward new human faces than old human faces, but six-month-olds spent significantly longer time to look at both new human faces and new monkey faces compared to the old ones. In other words, the youngest group preferred novel faces of two species, while the two older groups preferred to process the novel picture only for human faces.
The results suggest that, like the way younger brains can tell apart new speech sounds more easily than adults, the ability to recognize new faces diminishes with time as the "perceptual window" narrows. Early brain tuning, the researchers remind, however, does not indicate adults cannot use perception to learn how to discriminate a new class of stimuli and learn new languages or tell apart non-humans.
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Charles A. Nelson of University of Minnesota is also a co-author on this Science study. This research was supported in part by University of Sheffield, National Institutes of Health, John D. and Catherine T. MacArthur Foundation and the James S. McDonnell Foundation.
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