Despite a sound mind and fine ears, some 5% to 8% of preschoolers somehow confuse the sounds of consonants and struggle to talk coherently. Now scientists think they know why: The children's brains aren't fast enough to perceive rapidly changing sounds. The finding, reported in the current issue of Nature, may lead to earlier diagnoses and treatments of language disorders.
More than 2 decades ago, psychologists established that children with extremely poor language skills have a hard time making out certain syllables like "ba" or "da." The snag seemed to be the extremely brief sounds--just tens of milliseconds long--that distinguish one consonant from the next. Although studies of the children's sight and touch suggested that their brains could not process rapid changes in many kinds of sensory input fast enough, the mechanism remained a mystery.
A team led by Beverly Wright of Northwestern University and Michael Merzenich of the University of California, San Francisco, decided to see whether the brief sounds might be "masked" by other sounds in speech. The team played a split-second tone--the length of key sounds made by the tongue, lips, or palate--followed by a hissing noise. Normal kids had no trouble flagging the tone, but eight diagnosed with specific language impairment (SLI) failed to perceive it. Wright speculates that the hiss "erases" the tone before it can register in the brain.
"This is a very important finding," says Paula Tallal, a cognitive neuroscientist at Rutgers University. She suggests the test could be turned into a simple screen to look for budding speech disabilities in children kindergarten-age or younger. Such kids could then be given therapy sooner, such as computer games that extend the duration of consonant sounds and improve the language skills of children with SLI.
Although the finding may help diagnose the condition, not everyone is convinced that masking itself causes speech difficulties. "I'm not certain yet that they've found the root of the problem," says Bruce Tomblin, a speech-language pathologist at the University of Iowa. He suspects that masking and language difficulties may stem from other problems, such as a limited attention span. The Nature study, he says, "has just shown a correlation."