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Friday, May 17, 2002
We're all narrow-minded
By ROWAN HOOPER
It's a commonly held belief that we lose brain cells as we age. But, in fact, although our brains may not work as well when we get older -- learning becomes harder, memories fuzzier -- the number of cells they contain remains the same, about 28 billion. Scientists think the real problem is that the myelin sheaths around neurons wear down, like the tires on an old car, and consequently the brain can't move as fast.
A child's brain, on the other hand, is like a superabsorbant sponge. It will mop up anything -- rules not to cross the road, drink bleach or look at the sun through a telescope. At the same time, children will happily believe in things as dotty as Father Christmas and hellfire. Yet at birth, the brain is far from fully developed. Major changes occur in the brains of babies, changes that affect the function of the brain in future life.
For example, 6-month-old human babies can easily distinguish between individual humans and individual monkeys, but adults, while automatically able to recognize different humans, are almost completely unable to tell one monkey from another. At first sight this may seem like a wantonly bizarre fact, but it might help explain "perceptual narrowing." A good example of this phenomenon is our diminishing ability, as we grow older, to differentiate between different speech sounds if they don't occur in our mother tongue.
Perceptual narrowing is well-known in Japan. Under a certain age, infants can distinguish the sounds of R and L, but if young children are exposed only to the Japanese language, which does not distinguish between the two, the brain develops only one category for these sounds. This is why American passwords during the Pacific War were things like "rabble rouser" and "rubbernecker," words that a Japanese soldier would have trouble pronouncing.
As for the differentiation of monkeys' faces, the University of Sheffield's Olivier Pascalis and colleagues report this week in Science that while 6-month-old babies can do it, 9-month-old ones can't.
"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 the Institute of Child Health, University College London. "Face recognition is one example of the way we study how the brain is divided up into 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."
The paper also suggests that perceptual narrowing in face recognition and sound recognition are linked, part of a more general change in the neural architecture of the brain.
"The overlap between cognitive systems, such as language- and face-processing, is not that surprising, as they are both learned at the same time," said Pascalis in an e-mail interview. "Most of the time we are seeing the face that we are talking to. There is also a general idea that the brain is specialized differently in infants than in adults, i.e. the neural networks involved in language during infancy are not exactly the same as those in adults."
Pascalis and colleagues' experiment, called a visual paired-comparison procedure, assessed recognition abilities in babies and adults. Adults and 6-month-old and 9-month-old human babies were shown pairs of male and female Caucasian faces and the faces of monkeys (Macaca fascicularis). Of the two pictures, one had never been seen before and the other was familiar from a prior viewing. Recognition was detected by videotaping and assessing the participant's tendency to stare at the new face longer.
"Our work is revealing how experience influences face-recognition, but other factors, such as emotion, might also play a role," said Pascalis. "Several studies of depressed mothers, or children who were maltreated, are showing how experience is crucial for 'normal' development."
So children without the experience of love and affection may grow up without "normal" neural development. "The Jungle Book's" Mowgli, raised by animals, would probably have face-recognition and facial-expression systems poorly developed for interacting with humans.
However, with a staggering 10,000 billion connections between its neurons, predicting what the brain is capable of is never simple, as the case of John Sabunya illustrates.
Sabunya was abandoned as an infant toward the end of the 1986-1995 civil war in Ugandan. He was later spotted scavenging for food with monkeys, apparently having been sheltered and fed by them. The man who found him, Paul Wasswa, adopted the boy, who was then about 4. He was half-starved and unable to communicate. Despite an infancy apparently without human contact, the child learned to speak, and it turned out that he had a beautiful singing voice. He later toured in London with a choir raising money for African orphanages.
You can e-mail Rowan Hooper at firstname.lastname@example.org