SAN FRANCISCO - Behind the sweet, bright-eyed face of an infant listening to a parent's "baby talk" is a sophisticated analytical machine sifting through every sound for patterns and meaning.

The machine may even be equipped with perfect pitch -- a trait rare in older humans -- to help the baby acquire subtle but key knowledge about sound.

This picture, painted Monday by scientists, sheds new light on how infants may use several mechanisms to learn a language -- perhaps the most complex goal a human being can achieve.

"All these mechanisms are working at the same time," said Michael Brent, one of the researchers. Brent is a computer scientist at Washington University who studies natural language learning.

The scientists gave their reports in San Francisco at the annual meeting of the American Association for the Advancement of Science, which ends today.

Their findings contribute to an emerging picture of language development in which infants give different weight to any of several learning mechanisms at different times.

Besides the intrinsic interest in how humans learn language, the research is important because it may fuel advances in speech-recognition technology in computers. Also, it may help scientists in the field of "artificial intelligence" design computers that can analyze the grammar of an unknown language.

To understand this research, it's helpful to keep in mind that babies encounter a complicated and confusing world of sights, sounds, textures and other goings-on. One challenge they face is extracting meaningful information from what appears to them to be a wild symphony of syllables, words and sentences.

For example, a baby must figure out where the words in a long stream of speech begin and end.

The Washington University work focused on how babies under 15 months begin to sort this out. Brent and his colleagues found that the short, vibrant words of parental "baby talk" are key.

"Mothers instinctively use short utterances," said Brent, who conducted the work with Jeffrey Siskind of the NEC Institute of Princeton, N.J.

"Short utterances, especially single words, help speed up recognition of speech and discovery of new words," he said.

Brent's team recorded more than 200 hours of mothers' baby talk to their children when the kids were between nine and 15 months old. They traced what the babies had learned and found that many of the words came from the short utterances.

"The first words children tend to learn are words their mothers speak in isolation, suggesting that such isolated words may form a foundation for early vocabulary learning," he said.

The Washington University work is important because it shows that babies learn many individual words before they start to pull them out of long strings of language.

This challenges an idea that gained prominence in the 1990s that babies learn words as part of long strings of words.

Babies pick words out of long strings later in development, Brent said.

At the University of Wisconsin at Madison, psychologist Jenny Saffran has found that 8-month-old babies are "little statisticians," as she put it. In other words, they are skilled at tracking how often certain sounds follow other sounds in the language.

As they soak up sounds like sponges, they apply that statistical knowledge to their efforts to distinguish words from strings of sounds.

For example, by hearing the talk of parents, a baby eventually learns that the letter "t" often follows the letter "p" in English words. This helps the baby know that the phrase "pretty baby" can be separated into two distinct words.

Saffran also found that a baby's brain is wired to recognize perfect pitch. That's the ability to identify a musical note without the benefit of a reference point, an ability rare even among musicians.

This ability shows how infants and adults process sounds differently.

"What's interesting here is we (as infants) may not have dedicated hardware just for language," she said. "The structure is probably general to many complex forms of learning, including music."

Perfect pitch slips away from humans as they grow older, probably because it's no longer that useful as a learning tool, Saffran said. But while infants have it, this ability no doubt helps them learn subtle differences between similar-sounding words, she said.

Do babies prefer a mother's voice to a father's? Scientists haven't studied that, Saffran said. But, she added, babies do prefer higher-pitched voices.

In view of the findings, should parents speak more short utterances than long phrases when talking to their babies? The scientists don't know, but they suggested doing what comes naturally.

"Don't worry about how you speak to your baby," Brent advised. Without trying, parents who speak to their children will supply plenty of short and long utterances.

"The important thing," he said, "is just to speak to them."

At a scientific conference in San Francisco Monday, Washington University Professor Michael Brent and others presented new research on how babies learn to talk. Click photo for a larger image. (Andrew Cutraro/P-D)