The difference in language profiles of children with Autism Spectrum Disorder and Down Syndrome is not driven by non-verbal cognition

Background: Autism Spectrum Disorder (ASD) and Down Syndrome (DS) are among the most common types of neurodevelopmental conditions that have co-occurring language impairments. Usually, non-verbal IQ has been reported as one of the main predictors of language functioning in children with these conditions. Although language abilities of children with ASD and DS have been described in the previous studies, there is still a lack of direct comparisons of language profiles in the non-verbal IQ-matched groups of children with these disorders and, therefore, it is largely unexplored whether language difficulties in these populations are of similar or different origins. Aims: The study provided a direct comparison of language profiles in non-verbal IQ-matched children with ASD and DS at different linguistic levels (phonology, vocabulary, morphosyntax) in both production and comprehension and explored the influence of different psycholinguistic variables on accuracy. Also, the study assessed whether non-language factors (non-verbal IQ and age) influence language skills in both groups of children. Methods & Procedures: In total, 60 children participated in the study: 20 children with ASD, 20 children with DS, and 20 typically developing controls (7-11 year old; all groups were age-matched). The language testing included 7 tests from the Russian Child Language Assessment Battery, assessing expressive and receptive language skills at phonological, lexical, and morphosyntactic levels. Outcomes & Results: Overall, we revealed both similarities and differences in language profiles between children with ASD and DS. At the group performance level, children with ASD and DS were comparable in vocabulary and syntax but differed in phonological processing on which children with ASD had higher accuracy. Some psycholinguistic variables that influenced accuracy in language tests performance were present uniquely in the ASD group: for example, autistic children struggled more with verbs than nouns in naming; or comprehended sentences with canonical SVO word order more accurately than sentences with noncanonical OVS word order. In comparison to children with DS, in the ASD group non-verbal IQ was related to language skills in three out of seven tests with the evidence of positive association between them.Conclusions & Implications: This study provided new insights on the differences in language profiles of non-verbal IQ-matched children with ASD and DS and identified specific impairments related to linguistic levels and structural language characteristics in each group. These findings contributed to speech and language therapy strategies as they highlighted specific ‘linguistic deficits’ that should be targeted during intervention and therapy.