Specific language impairment (SLI) is a developmental language disorder that can affect both expressive and receptive language. SLI is defined as a "pure" language impairment, meaning that is not related to or caused by other developmental disorders, hearing loss or acquired brain injury.
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SLI is used to refer to problems in the acquisition and use of language, typically in the context of normal development. Whether it refers to individuals with normal overall cognitive development is controversial (Cohen, 2002).
Individuals with SLI exhibit problems in combining and selecting speech sounds of language into meaningful units (phonological awareness). These problems are different to speech impairments that arise from difficulties in coordination of oral-motor musculature (Cohen, 2002).
Symptoms include the use of short sentences, and problems producing and understanding syntactically complex sentences. SLI is also associated with an impoverished vocabulary, word finding problems, and difficulty learning new words, whereas the basic tasks for development of phonology and syntax are completed in childhood, vocabulary continues to grow in adulthood (Bishop, 1997 as cited in Cohen, 2002).
One of the hallmarks of SLI is a delay or deficit in the use of function morphemes (e.g., the, a, is) and other grammatical morphology (e.g., plural -s, past tense -ed). They omit function morphemes from their speech long after age-matched children with typical language development show consistent production of these elements.
Some researchers claim that SLI children's difficulty with grammatical morphology is due to delays or difficulty in acquiring a specific underlying linguistic mechanism. For example, Mabel Rice and Ken Wexler suggest that children with SLI have difficulty acquiring the rule that verbs must be marked for tense and number ("he walks", not "he walk"; Rice, 1994).
A second hypothesis is that these children have a deficit in processing brief and/or rapidly- changing auditory information, and/or in remembering the temporal order of auditory information. For example, Paula Tallal has found that some children with SLI have difficulty reporting the order of two sounds when these sounds are brief in duration and presented rapidly (Tallal, et al., 1985). Laurence Leonard suggests that these deficit may underlie difficulties in perceiving grammatical forms (e.g., "the", "is"), which are generally brief in duration (Leonard et al., 1997).
A third hypothesis is that children have poor short-term memory for speech sounds (e.g., Gathercole, 1998). Children with SLI perform worse than children with typical language skills on repeating nonsense words (for example, "zapanthakis"). In a number of recent studies short-term memory for speech sounds has been shown to correlate highly with vocabulary acquisition and speech production. This has led to the hypothesis that a primary function of this memory is to facilitate language learning.
A single nucleotide polymorphism (rs17236239) in a gene called CNTNAP2 may be associated with SLI.[1]
According to Bishop & Snowling (2004) among SLI children, about 50% will go on to experience reading difficulties and develop dyslexia.
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