Human Frontier - 10th Anniversary Brochure
Studying how babies learn to see
After spending two years in the USA thanks to an HFSP fellowship, Prof. Mark Johnson, a British researcher, set up a project with top researchers on the development of visual cognition in babies. When a child is born, how much will it depend on instinct as it learns about life? In other words, what functions can its brain perform spontaneously?
There is no easy answer to this apparently simple question. In fact, there are few brain activities that really start at birth and that can be analyzed as they develop. In this respect, face recognition is ideal for observation, because the baby has seen nothing similar inside its amniotic sac, and will be learning to recognize individuals and understand the meaning of facial expressions long before mastering the spoken language. "In fact, the notion that instinct and learning are in some way opposed has now been overtaken", explains Prof. Mark Johnson of the MRC Cognitive Development Unit in London. "It is wrong to think that the brain is prewired at birth and that it merely needs to record information in certain specialized areas. Certainly, the basic structural organization of nerve cells in the cerebral cortex—the part of the brain linked to complex thinking skills—is already established at birth. But the detailed pattern of connections between neurons arises only later, as a result of brain activity in the wake of experience. It is to some extent because of this growth of new connections that the human brain quadruples in volume between birth and adulthood."
Facial attraction • Several years ago, Prof. Johnson and Prof. John Morton established that even in the first half hour of life, babies will turn their head and eyes further to keep in view a schematic face—three black spots representing two eyes and a mouth—rather than other patterns (see picture on right). Since then, researchers believe that this instinctive attraction is the bait that ensures that what a baby sees in the earliest days includes faces. This visual input evokes brain activity, with the result that certain regions of the cortex begin to develop and specialize in processing complicated information—such as who the persons are, what sex they are, and what their expressions mean.
64 electrodes • Visualizing what is going on in a baby’s head is not as easy as with adults, since the use of MRI or PET scanners creates practical and ethical difficulties. Johnson’s group therefore uses a kind of hairnet made up of 64 contact electrodes, which is simply placed over the child’s head. Mark Johnson learnt to use this method in the USA, in the laboratory of Prof. Michael Posner and Prof. Mary Rothbart (Department of Psychology, University of Oregon, USA), where he went in 1990 for post-doctoral studies with the first batch of HFSP fellows. "Those two years were very important in my career", he now explains. "They gave me a chance to learn new techniques and to build up closer contacts with all the top specialists working on the development of visual cognition in babies."
Back in England, Johnson continued to cooperate closely with the lab of Prof. Posner and Prof. Rothbarth. He then set up a research project with his American colleagues and the groups of Prof. Daphne Maurer in Canada (McMaster University, Hamilton) and Prof. Scania de Schonen in France (Laboratory of Cognition and Development, Paris). In 1995, he obtained an HFSP research grant for the project Visual cognition in the human infant in relation to cortical development.
Always something new • When trying to recognize a face, an adult compares it with a prototype he or she has already built up in memory. The newborn baby does not yet have a prototype, and therefore has to accumulate a lot of visual experience before beginning to distinguish the subtle differences between human beings, among billions of possible variations. We often experience a tinge of pride when stared at by an unknown baby, who may be ignoring the faces of its own family. In fact, there is nothing to be proud of. Babies very often prefer to look at new faces rather than at those they already know. This attraction for novelty drives them to accumulate experience.
In order to measure the brain's response to a face, six-month-old children were fitted with a geodesic sensor net, which detects very low voltage changes on the surface of the head. Brain responses specific to faces were not seen until after 356 milliseconds - although they appear after only 128 millisencods in adults (arrows show the direction of the gaze). In babies a face produces a much stronger wave of activity than the sight of any other object. Indeed, experiments have shown that, to memorize a face, a one-month-old baby needs to look at it for about one minute. This learning time diminishes as the child grows older—girls, who usually mature a little earlier than boys, need on average less time than boys.
"Experiments carried out so far with babies suggest that this innate attraction for faces is prompted by a more primitive part of the brain, which is located under the cortex", concludes Johnson. "This instinct disappears little by little—it is inhibited by other neurons—as the baby gradually shapes his or her brain for face recognition.
Newborn babies are at most attracted by patterns representing a schematic face
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