This article considers how information regarding processing limitations in children with specific language impairment (SLI) might be extended to assist in early identification of toddlers at risk for language disorder. A brief review of the evidence for processing capacity limitations in SLI is provided, along with results from longitudinal studies of toddlers with late onset of language development. Preliminary findings are presented from an ongoing investigation of early lexical processing for 15 late talking toddlers and 15 controls whose performance was assessed on a novel word learning task. Assessment implications of a processing based account of language impairment are discussed.
AS CLINICIANS KNOW, our clinical practices are constantly evolving in response to changing perspectives and new information. Within our field, models of language impairment, assessment, and intervention have traditionally been derived from theories of normal language acquisition and use (Craig, 1982; Evans, 1996b). Current theories of language disorder, in particular accounts of specific language impairment (SLI), parallel present theories of normal language and can be grouped broadly into either competence or performance-based models. This distinction between competence and performance was originally introduced by Chomsky (1957) to distinguish an idealized speaker's knowledge of grammatical rules (competence) from the speaker's real time processing of language (performance) which additionally entails nonlinguistic factors such as attention and memory.
Competence-based accounts of SLI assume that language impairments are reflective of problems in the children's underlying grammar (Clahsen, 1989; Gopnik & Crago, 1991; Rice & Wexler, 1996). These theories differ with respect to which aspects of the child's grammar are posited to be impaired. They are similar, however, in that they all focus on characterizing the child's underlying linguistic knowledge at any given point in development. Alternatively, performance-based accounts of SLI assume that the language difficulties are secondary to cognitive/information processing deficits. These accounts can be grouped broadly into those that presume a specific processing mechanism is responsible for language disorder in these children and those that presume generalized deficits in processing capacity. For example, some investigators propose that specific deficits in phonological working memory (Gathercole & Baddeley, 1990) or temporal processing (Merzenich et al., 1996; Tallal et al., 1996) underlie language impairment. Other investigators propose that generalized processing capacity limitations, such as slower speed of processing (Miller, Kail, Leonard, & Tomblin, 2001; Leonard, 1998) or restrictions in working memory capacity (Ellis Weismer, Evans, & Hesketh, 1999; Montgomery, 2000a,b), account for these children's problems in acquiring and using language. The specific mechanism accounts differ in terms of whether or not language-specific deficits would be expected—the phonological working memory account makes language-specific claims, whereas the temporal processing account has been interpreted as a more generalized inability to process information of short duration regardless of modality (Tallal & Piercy, 1973a,b) or as a problem particular to processing rapidly changing acoustic cues in the speech signal (Merzenich et al., 1996). The accounts positing general information-processing deficits would necessarily implicate both verbal and non-verbal difficulties.
Given the known heterogeneity within the population of children with SLI, it is unlikely that a single deficit can explain the diverse pattern of language impairments observed (Bishop, Bishop, Bright, James, Delaney, & Tallal, 1999; Briscoe, Bishop, & Norbury, 2001; Ellis Weismer et al., 1999). Therefore, it is likely that for any given child or subgroup of SLI there may be multiple factors (e.g., rate of processing, phonological working memory limitations) contributing to the language disorder.