Past tense morphology in specifically language impaired and normally developing children

This study evaluates the input-processing deficit/single system and the grammar-specific deficit/dual system models to account for past tense formation in impaired and normal language development. We investigated regular and irregular past tense formation of 60 real and novel regular and irregular verbs in "Grammatical (G)-SLI'' children (aged 9:3 to 12:10) and morphological- or vocabulary-matched younger control children. The G-SLI children and language ability (LA) controls showed quantitatively and qualitatively different patterns of performance. The LA controls, but not the G-SLI children, showed a significant advantage of regular over irregular past tense marking for real and novel verbs. Past tense frequency affected the G-SLI children, but not the controls' production of regular verbs, even with stem access controlled for. The G-SLI children's production of regular forms was significantly lower than that of the control groups. Frequency and phonological properties had a similar and significant effect on the G-SLI and LA controls' irregular formation. The G-SLI children's irregular past tense production did not differ from that of the morphological controls, but was lower than that of the vocabulary controls. We argue that the dual mechanism/grammar-specific deficit provides a parsimonious explanation for normal and impaired performance, and suggest that grammatical computations underlying regular past tense formation in normal grammar are impaired (not missing) in G-SLI grammar.