The relationship between language and executive functions in adolescents with Down syndrome and fragile X syndrome

Individuals with fragile X syndrome (FXS) and Down syndrome (DS) have significant and pervasive challenges in language (and more specifically grammar) and executive functions (EFs). While these aspects of development are linked in autism and developmental language disorder, there has not been an investigation into this in FXS and DS. Thus, the purpose of this study was: 1) to evaluate the feasibility of experimental tasks for language and EFs, 2) to test if there are differences in language and EFs in DS and FXS, and 3) to test if EFs are related to grammatical abilities in DS and FXS within and between groups.

Participants included 21 boys with FXS and 25 participants with DS (n = 9 females) between 9-17 years of age; groups were matched on chronological age (variance ratio = 1.13; d = 0.04, p = 0.897) and were similar on nonverbal IQ and vocabulary. Participants completed lab-based assessments including standardized assessments of nonverbal IQ and vocabulary, experimental measures of grammar (i.e., grammatical judgment and sentence imitation), three experimental executive function tasks, and a parent report of executive functions.

While there were participants who could not complete the tasks, overall the feasibility was high (72-91% participants completed the tasks). Wilcoxon rank-sum tests revealed no significant group differences in experimental grammar or EF tasks. In contrast, large differences emerged on parent-reported EFs, with greater impairment in FXS for shifting and inhibition. We used generalized linear regression models with Gaussian and binomial distributions to examine the relationships between EFs and grammatical abilities. We found that only working memory significantly predicted grammatical judgment.

Participants with DS and FXS showed similar grammatical production and comprehension skills, contrasting with prior studies that relied on standardized testing and found more impaired production skills for children and adolescents with DS. Our sentence imitation task highlighted expressive grammar skills in DS, while grammaticality judgment posed challenges as a measure of grammar comprehension. Feasbility was good for all tasks, but there was a range, and younger participants in particular seemed to struggle with some of the tasks. The contrast in group differences between experimental and parent-reports of EFs calls into question whether the two measure EFs in a similar manner. Lastly, our study suggests that the language-EF relationships in intellectual disabilities may diverge from patterns documented in neurotypical development and language impairment without intellectual disability.