Through the time they're about six months old, babies have the ability to discriminate between not only human faces but those of monkeys, too, a new study has found. That ability wanes by an infant's ninth month, however, suggesting that by then the brain's face recognition wiring is becoming more species-specific.
"Unless they're exposed to those faces, they may never be able to discriminate between monkey faces as well as they distinguish between human faces," says Charles Nelson, a psychologist at the University of Minnesota and a co-author of the study.
What replaces that extraordinary flexibility is a "prototype" composite of human faces with which the growing baby compares the new ones it encounters, says Olivier Pascalis, a brain researcher at the University of Sheffield, England, and lead author of the study, which appears in tomorrow's issue of Science.
Paul Quinn, a psychologist at Washington and Jefferson College and an expert in facial recognition, says the new study supports the notion that for a few months after birth humans are sensitive to all kinds of faces. "Experience seems to narrow down that broad sensitivity, and we become particularly well tuned to process faces that we are experiencing in the environment," he says.
That kind of "perceptual narrowing" applies to sound and language recognition, too, Quinn says, and probably all other species-specific sensory input. "Babies in the first year of life become human experts based on their experience with human stimulation."
To demonstrate that young infants, but not adults, can distinguish between monkey and human faces, the researchers showed groups of each pictures of primates and people and measured their brain waves. They also repeated the experiment with the pictures inverted, which impairs the ability to recognize figures.
In both experiments, the young infants responded as if they were distinguishing between both monkey and human faces, while the adults could only differentiate between those of people.
The researchers then showed pairs of photos -- of people and of macaque monkeys -- to three groups: 30 six-month-old babies, 30 nine-month-old babies, and 11 men and women with no special history with primates. All the babies were healthy and full-term at birth.
One of the faces in the pair was shown first to make it familiar. Humans are attracted to novelty. Not surprisingly, then, the youngest babies spent about 1.0 to 1.5 seconds longer, on average, looking at both new faces, suggesting they distinguished between the monkeys.
However, the older babies spent more time looking at the human face only -- as would be expected if they could no longer distinguish monkey features. Adults had the same pattern as the nine-month-old infants, staring longer at the new human face but spending the same time on the monkeys.
Nelson says the study shows only that babies have the ability to differentiate between primate faces, not that they can somehow recognize them.
Although the findings are largely theoretical, they do have a few practical implications, Pascalis says. For instance, some babies are born with cataracts clouding their eyes and preventing them from seeing faces. Surgeons debate when to remove them.
However, as the new work indicates, "if you have the cataracts for several months and no visual input, the face system is different than [babies] with normal vision," Pascalis says. "You should have this kind of cataract operation as soon as possible."
The work may also be important for children with autism, those who suffer physical abuse and those whose mothers are severely depressed, Nelson says. Each has been shown to affect the ability to identify expressions.
Nelson and his colleagues are now conducting a study in a Romanian orphanage to look at how well children there recognize and differentiate facial expressions.
What To Do: To find out more about infant development, try the National Institute of Child Health and Human Development or the National Center for Infants, Toddlers & Families.