Babies prune their focus: perception narrows toward infancy's end.

Rather than crawling inexorably toward a better appreciation of the world around them, infants take a perceptual step backward before their first birthday, a new study indicates. That reversal, ironically, paves the way for advances in thinking later in childhood.

When shown videotapes of monkeys' faces that either matched or clashed with sounds being made by the animals, 4- and 6-month-olds preferred to look at matches, whereas 8- and 10-month-olds displayed no preference, say David J. Lewkowicz of Florida Atlantic University in Boca Raton and Asif A. Ghazanfar of Princeton University. Looking preferences in the younger babies denoted an awareness of associations between faces and vocalizations, the researchers assert.

Younger infants probably noted when facial movements were synchronized with vocalizations, the two psychologists assert in the April 25 Proceedings of the National Academy of Sciences. The older infants ignored the basic phenomenon of synchrony because they had entered a phase of looking for more-complex features of human faces and voices, the scientists propose.

"We're tapping into a transition period in late infancy when ... it's more difficult to perceive links between different sensations," Lewkowicz says. Constrictive phases characterize development of various types of perception, in his view.

For instance, other researchers have found that, between 6 and 10 months of age, infants improve at discriminating among sounds in their native languages and among different people's faces. During that same time, youngsters become worse at telling apart foreign-language sounds and other species' faces, such as those of monkeys (SN: 5/18/02, p. 307).

Lewkowicz and Ghazanfar suspected that a comparable form of perceptual narrowing occurs as babies learn about critical relationships between different sensations, such as sights and sounds. The experiment consisted of 33 infants at 4 months of age, 57 at 6 months, 54 at 8 months, and 32 at 10 months.

Each baby sat in front of two adjacent video monitors and completed four 1-minute trials. Flashing lights drew each infant's attention to the monitors as pairs of videos showed monkeys' faces making either coo or grunt calls. Characteristic lip and facial movements for each call were accompanied either by the sound of the same call or by the sound of the other call.

At the two youngest ages, infants looked substantially longer at faces that made matching calls than they did at faces that emitted mismatched calls. At the two oldest ages, infants looked at instances of matched and mismatched calls for about the same amount of time.

It's already known that, after 3 months of age, babies associate people's faces with their vocalizations, Lewkowicz notes. As perceptual experience in this vital social realm mounts during infancy, youngsters temporarily lose the broader capacity to recognize links between the facial movements and vocalizations of other species, he proposes.

Olivier Pascalis, a psychologist at the University of Sheffield in England, agrees. "There are clearly a series of transitions going on that move babies from a broad perceptual system toward a specialized one," he says.

Lewkowicz plans to examine at what age, after 10 months, sensory-matching ability reappears.