Language Impairment, Developmental

It has been estimated that approximately thirteen percent of all children have some form of language impairment (Beitchman et al. 1986a). The most common known causes of developmental language impairments are hearing loss (including intermittent hearing loss resulting from chronic otitis media), general MENTAL RETARDATION, neurological disorders such as lesions or epilepsy affecting the auditory processing or language areas of the brain, and motor defects affecting the oral musculature. Many other developmental disorders, such as pervasive developmental disability (including AUTISM), attention deficit disorder, central auditory processing disorder, and Down's syndrome, may include delay in language development. In addition to these known causes of developmental language impairment, a recent epidemiological study of monolingual English-speaking kindergarten children in the United States found that approximately 8 percent of boys and 6 percent of girls have a significant developmental language impairment of unknown origin, referred to as specific language impairment (SLI; Tomblin et al. 1997). This epidemiological study also showed that the clinical identification of language impairments remains low. Only 29 percent of the parents of children identified as having SLI had previously been informed that their child had a speech or language problem.

The differential diagnosis of developmental language impairments is based on behavioral evaluations that include audiological, neurological, psychological, and speech and language testing. Developmental language disorders are divided into two basic categories, expressive language disorder and mixed receptive-expressive language disorder, a disorder encompassing both language comprehension and production deficits (DSM-IV. 1994). Comprehension or production problems may occur within one or more of the components of language, including PHONOLOGY, morphology, SEMANTICS, or SYNTAX. Problems with PRAGMATICS, that is, conversational skills, also occur frequently. A high proportion of children with developmental language disorders also have concomitant speech articulation defects, that is, they have difficulty clearly and correctly producing one or more of the speech sounds of their language. However, speech articulation defects and developmental language impairments can occur independently of each other.

Developmental language impairment has been shown to be a risk factor for other childhood disorders. For example, epidemiological studies showed that children referred to child guidance clinics for a variety of social and emotional conditions were found to have a higher-than-expected incidence of developmental language disorders (Beitchman et al. 1986b). Conversely, children diagnosed with developmental language disorders also have been found, upon examination, to have a preponderance of behavioral and emotional disorders (Cantwell, Baker, and Mattison 1979). Longitudinal research studies that have followed children with early developmental language impairments prospectively from the preschool though elementary school years have demonstrated a striking link between early developmental language impairments and subsequent learning disabilities, especially DYSLEXIA, a developmental reading disability (Aram et al. 1984; Bishop and Adams 1990; Rissman, Curtiss, and Tallal 1990; Catts 1993). Research that has compared children classified as SLI with those classified as dyslexic has shown that both groups are characterized by a variety of oral language deficits, specifically phonological analysis deficits (Liberman et al. 1974; Wagner and Torgeson 1987; Shankweiler et al. 1995). Whether the phonological deficit derives from speech-specific mechanisms, or from more basic acoustic processing deficits, has been the focus of considerable research and theoretical debate.

Phonological processing deficits are generally accompanied by central auditory processing disorders, particularly in the areas of AUDITORY ATTENTION and serial memory. These processing problems may result from a more basic impairment in the rate of neural information processing, specifically a severe backward masking deficit (Tallal, Miller, and Fitch 1993a; Tallal et al. 1993b; Wright et al. 1997). Tallal and colleagues have shown that children with phonologically based speech, language, and READING disorders need significantly more neural processing time (hundreds of milliseconds instead of tens of milliseconds) between brief, rapidly successive acoustic cues in order to process them correctly. This slowed processing rate has a particularly detrimental effect on phonological processing. Many acoustic changes occurring within syllables and words, necessary to distinguish the individual phonological elements (speech sounds), occur within the tens-of-milliseconds time window. Research has demonstrated that perception of those speech syllables that incorporate rapidly successive acoustic cues is most problematic for these children (see Tallal et al. 1993a for review).

These findings linking slow auditory processing rate and phonological processing deficits to developmental language and reading disorders have recently led to the development of novel remediation strategies for the treatment of developmental language-learning impairments. In a treatment-controlled study, using speech that was acoustically computer modified to extend and amplify the rapidly successive components, it was shown that intensive, individually adaptive daily training with computer-based exercises resulted in highly significant gains in auditory processing rate, speech discrimination, and language comprehension of syntax and grammar (Merzenich et al. 1996; Tallal et al. 1996).

There also have been considerable advances made in understanding the specific linguistic deficits of these children. Research has demonstrated particular difficulty with components of syntax and morphology (see Leonard 1998 for review). The results of longitudinal studies have shown, however, that children with developmental language impairments develop the linguistic structures of language along a similar linguistic trajectory to that observed in normal younger children. There is little evidence that these children make linguistic errors that are deviant or substantially different from younger children at the same stage of language development. Rather, children with language impairments take much longer to progress through the stages of normal language development (Curtiss, Katz, and Tallal 1992). A similar pattern of delay, rather than deviance, occurs across most populations of children with developmental language impairment. Looking cross-linguistically at children with developmental language impairments learning different languages in different countries, it also has been shown that whatever linguistic structures are unstressed or of weak phonetic substance in a particular language are the most difficult for children to learn and also the most delayed in children with language impairments (Leonard 1992). That the same order of development occurs across populations and language environments gives strong support that there is a potent metric of sheer difficulty (whether representational or perceptual) that is imposed on the learning of linguistic structures and contents.

There is growing evidence from family and twin studies that developmental language impairments may aggregate in families and may be genetically transmitted. Twin studies have shown a high concordance rate (heritability) for measures of phonological analysis (Bishop, North, and Donlan 1995). As a group, infants born into families with a positive history of developmental language impairment demonstrate longer processing times than matched infants born into families with a negative family history for SLI. When followed prospectively, the processing rate established at six months of age has been shown to predict the rate of language development, with the infants showing impaired processing rate subsequently being most likely to be delayed in language development (Benasich and Tallal 1996). It also has been found that adults with dyslexia, or who have a family history of language-learning impairments, show significant psychoacoustic deficits, particularly a slower auditory processing rate. These adults also show poorer phonological analysis abilities when compared with matched adults who are family history - negative for developmental language learning disorders (Tomblin, Freese, and Records 1992).

Recently, brain imaging technologies also have been used to examine the neurobiological basis of developmental language impairment. Electrophysiological studies support the results of behavioral studies showing specific deficits in acoustic analysis and phonological processing (Kraus et al. 1995). Results from MAGNETIC RESONANCE IMAGING (MRI) show that the pars triangularis (Broca's area) is significantly smaller in the left hemisphere of children with SLI and that these children are more likely to have rightward asymmetry of language structures. The opposite pattern is seen in children developing language normally (Jernigan et al. 1991; Gauger, Lombardino, and Leonard 1997).

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-- Paula Tallal

References

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Tallal, P., S. L. Miller, G. Bedi, G. Byma, X. Wang, S. S. Nagarajan, C. Schreiner, W. M. Jenkins, and M. M. Merzenich. (1996). Language comprehension in language-learning impaired children improved with acoustically modified speech. Science 271:81-84.

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Tomblin, J. B., N. L. Records, P. Buckwalter, X. Zhang, E. Smith, and M. O'Brien. (1997). The prevalence of specific language impairment in kindergarten children. Journal of Speech, Language, and Hearing Research 40:1245-1260.

Wagner, R. K., and J. K. Torgeson. (1987). The nature of phonological processing and its causal role in the acquisition of reading skills. Psychological Bulletin 102:192-212.

Wright, B. A., L. J. Lombardino, W. M. King, C. S. Puranik, C. M. Leonard, and M. M. Merzenich. (1997a). Deficits in auditory temporal and spectral processing in language-impaired children. Nature 387:176-178.

Language Impairment, Developmental

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