Specific Language Impairment

1. One of the notable criteria of specific language impairment in children is that their assessed cognitive abilities (i.e., non-verbal/performance IQ) must fall within the normal range (cf. Benton, 1964; Eisenson, 1968; Kamhi, 1991; Leonard, 1982, 1987, 1998; Plante, 1998; Rees, 1975; Stark & Tallal, 1981; Tallal, 1975, 1988; West, 1962). However, a critical look at the research in this area shows that while this is accurate, often times the assessed cognition of children with SLI is actually less than that of their normally developing peers. While it may be accurate to note that children with SLI fall within the normal range of non-verbal/performance cognitive abilities, there is, however, heterogeneity, they typically fall below the population mean, and there are notable differences between them and their normally developing peers (cf. Swisher & Plante, 1993; Swisher, Plante, & Lowell, 1994). The purpose of this research was to conduct an objective, quantitative, evidence-based, statistical meta-analysis of the reported cognitive abilities of children with SLI and their normally developing counterparts. For this research, studies investigating morpho-syntax or non-word repetition abilities of children with specific language impairment and normally developing controls, published in either the Journal of Speech, Language, and Hearing Research, or the Journal of Communication Disorders over the past 20 years or so, were utilized in a statistical, effect size, meta-analysis of the reported assessed non-verbal/performance cognitive abilities of the experimental and control research participants. Where possible, available data on research participants’ assessed non-verbal cognitive performance were extracted and employed in a meta-analysis employing an effect-size strategy. These data were utilized in the calculation of statistical effect sizes (ES) (i.e., Cohen's d) regarding the assessed non-verbal/performance cognitive abilities of the children with SLI and their normal control counterparts. Presently, the overall calculated effect size is: d = -0.81 (95% CI = -.91 to -0.71). That is, an effect size of -.81, with a 95% confidence interval of: -.91 to -.71. The minus sign of the effect size indicates that the ‘experimental group’ (i.e., children with SLI) fall below the ‘control group’ (i.e. normally developing children) for the domain of non-verbal/performance cognition. An interpretation of these data is that while children with SLI by definition place within the ‘normal range’ of assessed cognitive abilities, they nonetheless score 0.81 standard deviation below their normal controls for this domain; and 95% of the time they fall .71 to .91 standard deviations below their normal controls. Considering the standard score equivalence standard deviation of 15 for cognitive evaluation, this means that the ‘typical’ child with SLI falls some 12 performance ‘IQ points’ below the ‘typical’ normally developing control child. Though ‘normal’ -- they are quite different. Cohen (1988) characterized effect sizes as: "small, d = .2," "medium, d = .5," and "large, d = .8". By that standard, the overall ES of d = -0.81 found here is very notable. Effect sizes can also be interpreted as the average percentile standing of the average experimental participant relative to the average control participant. For example, an ES of d = 0.0 indicates that the mean of the experimental group is at the 50th percentile of the control group. An ES of d = -0.8 indicates that 79 percent of the control group performed better than the experimental group. In the present research, the effect size of d = -0.81 indicates that approximately 80% of the control group population of normally developing children scored higher than the experimental group population of children with specific language impairment for the variable of performance/non-verbal intelligence. Further, an effect size is the degree to which the ‘null hypothesis’ -- that is, the ‘no difference hypothesis’ -- is ‘false.’ In this particular case it addresses the degree to which the following null hypothesis is false: ‘Children with specific language impairment and normally developing children are from the same population as regards their non-verbal/performance-based cognition.’ Given the reported effect size here, the population-level null hypothesis of no difference between the non-verbal/performance cognition of children with SLI and those developing language normally cannot be supported. The importance of the research presented here is that it adds significant information to the database as regards SLI. Perhaps most importantly, it objectively examines the question of – "How specific is ‘specific language impairment’?" In that, this research is a critical and evidence-based examination of potential similarities and differences for SLI, and normal development vis-à-vis cognition. It addresses the argument that SLI may not be so ‘specific’ to difficulties in the linguistic domain alone. The results demonstrate that not only are children with SLI different from normally developing children with regard to language abilities, but they also differ from them, though perhaps more subtly, with regards to cognitive performance. Newbury, Bishop, and Monaco (2005, p. 528) stated that “specific language impairment is a failure to acquire age-appropriate language despite normal non-verbal intelligence and otherwise typical development.” The results of the research presented here, clearly call for an appropriate level of reconsideration/reconceptualization of the domain of non-verbal/performance cognition and SLI. At the very least perhaps, it needs to be recognized as an important though conceivably ‘sub-clinical’ characteristic. Even so, they go on to argue that “…we need to use measures of the underlying cognitive basis of SLI in our quest for genotype-phenotype relationships.” Appropriately so, they point out that SLI is a heterogeneous condition clinically, and likely so in etiology, and pathogenesis. It should be noted that, data from Miller, Leonard, Kail, Zhang, Tomblin, and Francis (2006) were not included in this project, as they more exactingly controlled for cognition by specifically matching experimental (SLI) and control (Normal Language) research participants for performance IQ. This research tactic does provide a markedly better control for the variable of cognition in research on specific language impairment. However it may also lead to biased sampling from the population of children with SLI (cf. Swisher & Plante, 1993; Swisher, Plante, & Lowell, 1994) . Specific Language Impairment & Cognition: A Meta-AnalysisMichael W. CasbyCommunicative Sciences & DisordersMichigan State Universityimail: casby@msu.eduAmerican Speech-Language-Hearing Association National Convention, 2008 Normally Developing Specific Language Impairment -3 -2 -1 1 2 3 0 Effect Size Graphic representation of population effect size of SLI and non-verbal cognition in children (d = -.81 (95% CI -.91 to -.71).