MINNEAPOLIS / ST. PAUL -- How good are you at recognizing the faces of monkeys? Chances are, you were very good at six months of age, but by nine months you were only good--or at least fast--at discriminating between faces of people. That's the conclusion of a study by researchers at the University of Minnesota and two English universities, who say it provides evidence that the brain's ability to perceive faces normally narrows as infants develop. The findings may help guide the treatment of people who suffer impaired ability to recognize faces or read emotions from facial expressions. The study will be published in the May 17 issue of Science.
The work tests an idea of Charles Nelson, Distinguished McKnight University Professor of Child Development, Neuroscience and Pediatrics and co-director of the Center for Neurobehavioral Development at the University of Minnesota, who is also an author of the paper. Nelson had proposed that as infants gain experience viewing faces, their brains--especially a certain area of the cerebral cortex known as the fusiform gyrus--"tune in" to the types of faces they see most often and tune out other types. This implied that younger infants, who have not had enough experience to become specialized in discriminating human faces, ought to do better than older infants or adults at telling faces of other species apart.
In tests of adults, 6-month-olds and 9-month-olds by Olivier Pascalis of the University of Sheffield, England, and Michelle de Haan of University College London, the younger infants outperformed both other groups in distinguishing the faces of monkeys. All groups were able to distinguish human faces from one another. In previous work, Pascalis and a colleague had shown that adult monkeys were better able to distinguish monkey faces than human faces. Both studies illustrate the same pattern, namely that because primates tend to be most familiar with faces of their own species, they learn to distinguish those types of faces but not others, said Nelson.
"I believe that the brain has the potential to become specialized to recognize faces, but this specialization occurs only with experience in viewing faces," said Nelson. "Experience with faces is crucial to driving the system that allows normal face recognition and emotional 'reading' skills. This is contrary to the views of researchers who think the ability to recognize individual faces is innate. If that were so, then the adults and the 9-month-olds would have been able to tell the monkey faces apart as easily as they distinguished human faces."
None of this means that the ability to distinguish faces of a different species can't be learned, said Nelson. People who regularly see or work with chimpanzees, monkeys or other species learn to tell them apart. But primates are not born with the ability to do this; they are born only with the ability to learn it.
"It's like learning to distinguish different species of birds or different models of cars," said Nelson. "We're not born with the ability to do it--just the ability to learn to do it."
Such "perceptual narrowing" may signal a general change in neural networks that is involved in early cognition, Nelson said. "We're interested in what this means in neurological terms," he said. "For example, we don't know why this particular area of the brain--the fusiform gyrus--gets the assignment of distinguishing faces."
Work by Daphne Maurer at McMaster University in Ontario is consistent with this model, he said. Her work showed that when babies with cataracts had the cataracts removed, their general visual acuity improved rapidly and dramatically, although their ability to recognize faces appeared to be somewhat delayed--due, perhaps, to not having had experience with faces at a particularly sensitive point in time. Similarly, Nelson recalled research showing that adults are better at recognizing faces from their own race than faces representing races with which they had had less experience. In contrast, children did better at recognizing faces from almost any race and didn't show the same deficit as did adults in recognizing faces from unfamiliar races. Researchers studying speech development have shown similar results. At 6 months, infants can discriminate sounds of nearly all languages, but between 9 and 12 months they become "specialized" in discriminating the sounds of their native language, Nelson said.
Nelson and his colleagues are expanding their study to include infants younger than 6 months and older than 9 months in order to pinpoint the changes in face recognition abilities. He said such research may help children born prematurely with retinopathy, children born with cataracts, or babies born with damage to the area of the brain that will become specialized for recognizing faces (i.e., the fusiform gyrus). Premature infants, for example, may need "more proactive work to help them learn to recognize people," Nelson said. Other implications of the work involve children who were maltreated, were raised by a depressed mother or who have autism. Such children may have trouble reading emotions on faces, with obvious and sometimes severe social repercussions.
"Perhaps, by understanding how the ability to recognize faces develops, we can find ways to help these individuals," Nelson said.
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