Language disorder tied to sound perception.

The conversational stream of daily life may flow by so quickly that it drowns out the ability of some children to distinguish discrete sounds and words and, as a result, to make sense of speech, a new study suggests.

An impairment of this type may set the stage for specific language impairment (SLI), a marked inability to use and understand speech that occurs in as many as 1 in 20 children, reports a research team led by neuroscientist Beverly A. Wright of Northwestern University in Evanston, Ill.

"This study provides a basis for early identification of [specific language impairment] and helps us to define the condition more precisely," says coauthor Michael M. Merzenich of the University of California, San Francisco.

The finding also coincides with evidence that SLI involves a broad range of problems, such as difficulties in pronouncing sounds of all kinds (SN: 2/4/95, p. 70). Other investigators, however, theorize that SLI stems from disturbances in brain circuits devoted specifically to grammar.

Wright's group studied eight children diagnosed with SLI and eight youngsters who displayed good language skills. The participants averaged 8 years of age. Each listened to a brief tone that was presented just before, during, or just after either of two "masking" sounds, one of a frequency similar to the tone and the other of a contrasting frequency.

In similar-frequency masking trials, language-proficient kids found it easiest to detect a tone that preceded the masking sound and hardest to detect a tone that occurred simultaneously with it.

In contrast, the researchers found, children with SLI could not hear any of the test tones unless the tones were substantially louder than those played for the control children.

Moreover, SLI-diagnosed participants had as much or more difficulty detecting a loud tone when it was presented before the noise as when it occurred during or after the noise.

Compared to controls, children with SLI were also less able to exploit a frequency contrast between the tone and a masking noise to enhance their detection of the tone, regardless of the order in which it was presented.

In children with SLI, such hearing deficits can obstruct perception of the individual speech sounds made by someone talking at a typical pace, Wright's group contends in the May 8 Nature.

The group's data elaborate on research by neuroscientist Paula A. Tallal of Rutgers University in Newark, N.J., which indicates that SLI involves a deficiency in perceiving the rapidly changing acoustic frequencies of certain consonant sounds, such as "ba" and "da."

"Wright's study suggests that in SLI, the stream of speech interferes with the perception of separate sounds and words," Tallal says.

Other studies of people with SLI find deficits in grammar understanding that are unlikely to spring from hearing impairments, argues linguist Myrna L. Gopnik of McGill University in Montreal. Subgroups of SLI arising from different causes may exist, she suggests.