|Specific Language Impairment|
Many different terms have been used to describe the disorder of childhood characterized by markedly delayed language development in the absence of any apparent handicapping conditions such as deafness, autism, or mental retardation. It is sometimes called childhood dysphasia, or developmental language disorder.
Much research since the 1960s has attempted to identify clinical subtypes of the disorder. These include verbal auditory agnosia and specific language impairment. Some children have a very precise difficulty in processing speech, called verbal auditory agnosia, that may be due to an underlying pathology in the temporal lobes of the brain.
The most prevalent sub-type of childhood language disorder, phonosyntactic disorder, is now commonly termed specific language impairment or SLI. These children have a disorder specifically affecting inflectional morphology and syntax.
Very little is known about the cause or origin (referred to as etiology) of specific language impairment, though evidence is growing that the underlying condition may be a form of brain abnormality, not obvious with existing diagnostic technologies: SLI children do not have clear brain lesions or marked anatomical differences in either brain hemisphere.
However, there is some indication of a familial pattern in SLI, with clinicians noting patterns across generations. It is more common in boys than girls. As of the 1990s, research suggests a possible genetic link, though there are still many problems in identifying such a gene.
Sometimes the siblings of an affected child show milder forms of the difficulty, complicating the picture. One of the major stumbling blocks is the definition of the disorder, because the population of children with language impairments is still much more heterogeneous than required to support a search for a gene.
Children with SLI usually begin to talk at roughly the same age as normal children but are markedly slower in the progress they make. They seem to have particular problems with inflection and word forms (inflectional morphology), such as leaving off endings as in the past tense verb form.
This problem persists much longer than early childhood, often into the grade school years and beyond, where the children encounter renewed difficulties in reading and writing. The SLI child has also been observed to have difficulties learning language “incidentally,” that is, in picking up a new word from context, or generalizing a new syntactic form.
This is in decided contrast to the normal child’s case, where incidental learning and generalization are the hallmarks of language acquisition. Children with SLI are not necessarily cognitively impaired, and are not withdrawn or socially aloof like the autistic child.
Some investigators have attributed the SLI child’s difficulty to speech sounds (a phonological problem), suggesting that inflection and word forms (morphology) such as endings are vulnerable because those items are so fleeting and unstressed in speech. It is not that the child is deaf in general, but that he has a specific difficulty discriminating speech sounds.
Other researchers have argued that this difficulty is not specific to speech but reflects a general perceptual difficulty with the processing of rapidly timed events, of which speech is the most taxing example. The left hemisphere of the brain seems to be specialized for processing rapid acoustic events, so perhaps the SLI child has a unique difficulty with that part of the brain.
Yet phonology does not seem to be the whole problem because the child may be quite good at articulation or speech perception per se. Instead, it is argued that the child may have a linguistic difficulty with morphology, going beyond the sounds themselves.
Cross-linguistic work supports a more refined perspective that suggests certain kinds of inflectional morphology, especially those associated with the verb, may be more likely to be disrupted than others. If so, that would suggest the problem is not just phonological and not just inflectional. Given the centrality of the verb to sentence structure, the difficulty causes pervasive problems.
Whatever the final identification of the linguistic problem, researchers are curious to discover how such a specific disorder could come about: is a language “module” of the brain somehow compromised in these children? The puzzle is that children with very precise lesions of the usual language areas somehow overcome those difficulties more easily than the SLI child who presents no such dramatic brain abnormalities.
The child with SLI becomes increasingly aware of his difficulties with language and may lose spontaneity and avoid conversation as he gets older. Intensive language intervention can allow these children to make considerable gains, with modeling of appropriate linguistic forms producing more gains than simply “enriching” the child’s language environment. Early identification is thus seen as very important for intervention.
One procedure for children aged 24 to 36 months asks parents to complete a standardized questionnaire in which they check off the vocabulary the child knows, and write down examples of the child’s two-word sentences. If the child has fewer than 50 words and no two-word sentences, that is an indication of risk for language disorder.
Estimates of true SLI vary according to the age of identification: some experts argue that as many as 10% of 2-year-olds may have a specific language impairment, but by age 3 or 4, that percentage drops considerably, presumably because some difficulties resolve themselves. The incidence in the general population is estimated at about 1%.