Past tense morphology in specifically language impaired and normally developing children

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