The Impact of Complexity on Sentence Comprehension in Children with Specific Language Impairment

In recent years, there has been growing evidence that children with specific language impairment (SLI) show poor sentence comprehension performance. However, factors determining sentence comprehension in SLI remain unclear. Limited processing capacities have been suggested as a potential cause of their syntactic problem (Ellis Weismer & Hesketh, 1996; Montgomery, 2000), but this hypothesis has not been directly addressed by manipulating inner sentence complexity. This study aims at investigating the limited capacity theory of language impairment in a sentence comprehension task, by manipulating various aspects of inner sentence complexity, as defined by the Capacity Constrained Comprehension theory (Just & Carpenter, 1992).

Fifteen children with SLI, 15 age-and-IQ-matched controls, and 15 controls matched for receptive lexical and grammatical abilities performed an auditory sentence comprehension task in which lexical frequency, sentence length, and the type of relative clause (subject- or object-relativized clauses) were manipulated.

Results showed a main effect of group (F(2,42)=25.40, p<.001): children with SLI performed worse than language-matched and age-matched control groups. Main effects of relative clause type (F(1,42)=198.37, p<.001) and sentence length (F(1,42)=46.29, p<.001) were also found, showing overall better performances for sentences with a subject rather than an object relative clause and for short rather than long sentences. However, the SLI group was affected to the same extent by these variables as the control groups. At the same time, a significant group-by-frequency interaction effect was found (F(2,42)=4.03, p<.05), revealing that only the SLI group showed poorer sentence comprehension performance for sentences containing low frequency words.

These data confirm the severity and specificity of sentence comprehension difficulties in children with SLI (e.g., van der Lely 2005). Given that our SLI group showed impaired sentence comprehension performance relative to younger children performing at the same level on a standardized test of grammatical comprehension, it appears that processing full reversible relative clauses is especially difficult for children with specific language impairment. Moreover, SLI children’s sentence comprehension abilities are particularly affected by the presence of low frequency but familiar words, even when compared to children matched on vocabulary knowledge.