Psycholinguistic Approach

Psycholinguistic research has centered on describing the linguistic profiles of children with LLI in an attempt to determine the areas in which they may be deficient in comparison to typically developing children. The component parts of language (phonology, morphology, semantics, syntax, and pragmatics) have each been studied extensively. These studies have raised several central questions.

Research has focused on whether those with LLI comprise a homogeneous group of individuals all manifesting a similar disorder or distinct subgroups of individuals with each group representing a different pattern of linguistic deficits. Although considerable research has focused on developing a detailed nosology of what distinct subgroups may be contained within the broader diagnostic classification of LLI, few studies have shown that an individual's classification within a subgroup remains stable over time.

Cross-linguistic research approaches have been used in an attempt to identify universal patterns of deficits that occur in children with LLI independent of the child's native language. For the most part, these studies have failed to identify patterns of linguistic deficits that apply across a variety of languages. For example, children with LLI who are struggling to learn English have been shown to have particular difficulty learning specific morphologic structures such as past tense. However, this pattern of linguistic deficit has not been found in studies of children with LLI learning other languages.

Another major research question that has been addressed using the psycholinguistic approach is whether individuals with LLI are basically (1) delayed in language acquisition, but otherwise developing language structures normally; or (2) deviant, developing abnormal language structures rarely encountered in persons who acquire language normally. The majority of research studies that have addressed the delay versus deviance issue have involved too few subjects to identify differences among distinct subgroups of children. In addition, until recently such studies did not follow a longitudinal research design. However, results have now been reported from a few large-scale longitudinal studies. In these studies change over time was directly assessed in subjects with a variety of language acquisition profiles. The outcomes of these studies demonstrate that, for the most part, the language acquisition profiles of children with LLI parallel those seen in normally developing, albeit younger, children. These findings are more compatible with delay than with deviance. On the other hand, the results of long- term follow up (into the teenage or adult years) of individuals in whom LLI was diagnosed in childhood are incompatible with the idea that LLI is primarily a delay in otherwise normal language development. These longitudinal studies demonstrate that rather than resolving over time, early oral LLIs may either persist throughout development or appear to resolve, only to be replaced later in development by other language-based learning disabilities such as reading, writing, and spelling deficits. This is particularly the case for children who demonstrate difficulty with both receptive and expressive aspects of language learning, specifically in the areas of phonology and syntax.

Psycholinguistic studies of children with LLI can be extremely difficult to interpret from a neuroscience perspective. Although developmental linguistic deficits have been hypothesized to be attributable to deficits in learning higher order linguistic rules, or to a defective "linguistic-specific brain module", it is also possible that linguistic deficits could result from more basic sensory, perceptual, motor, and/ or cognitive deficits that are prerequisite to developing more complex processes such as speech and language. Neuropsychologic research has taken a more reductionistic approach to the study of language development and disorders. More recently, especially with the advent of new imaging technologies and gene linkage approaches to the study of heritable factors involved in LLI, cognitive neuroscientists have increasingly focused on the neurobiologic basis of LLI.

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