Contributions of phonological and verbal working memory to language development in adolescents with fragile X syndrome

J Neurodevelop Disord (2011) 3:335–347 DOI 10.1007/s11689-011-9095-2 Contributions of phonological and verbal working memory to language development in adolescents with fragile X syndrome Elizabeth I. Pierpont & Erica Kesin Richmond & Leonard Abbeduto & Sara T. Kover & W. Ted Brown Received: 13 March 2011 / Accepted: 27 September 2011 / Published online: 13 October 2011 # Springer Science+Business Media, LLC 2011 Abstract Fragile X syndrome (FXS) is the most common inherited cause of intellectual disability. Although language delays are frequently observed in FXS, neither the longitudinal course of language development nor its cognitive predictors are well understood. The present study investigated whether phonological and working memory skills are predictive of growth in vocabulary and syntax in individuals with FXS during adolescence. Forty-four individuals with FXS (mean age=12.61 years) completed assessments of phonological memory (nonword repetition and forward digit recall), verbal working memory (backward digit recall), vocabulary, syntax, and nonverbal cognition. Vocabulary and syntax skills were reassessed at a 2-year follow-up. In a series of analyses that controlled for nonverbal cognitive ability and severity of autism symptoms, the relative contributions of phonological and working memory to language change over time were investigated. These relationships were examined separately for boys and girls. In boys with FXS, phonological memory significantly predicted gains in vocabulary and syntax skills. Further, verbal working memory was uniquely associated with vocabulary gains among boys. In girls with FXS, phonological and working memory skills showed no relationship with language change across the 2-year time period. Our findings indicate that, for adolescent boys with FXS, acquisition of vocabulary and syntax may be constrained by the ability to maintain and manipulate phonological representations online. Implications for the identification and treatment of language disorders in this population are discussed. The present study is the first to identify specific cognitive mechanisms contributing to language growth over time in individuals with FXS. Keywords Fragile X syndrome . Language . Phonological memory . Working memory . Nonword repetition . Digit span Introduction E. I. Pierpont (*) Department of Psychology, University of Wisconsin–Madison, 1202 W. Johnson Street, Madison, WI 53706, USA e-mail: W. T. Brown Institute for Basic Research in Developmental Disabilities, Staten Island, NY, USA E. K. Richmond : S. T. Kover Waisman Center, University of Wisconsin–Madison, 1500 Highland Avenue, Madison, WI 53705, USA L. Abbeduto UC Davis MIND Institute, University of California-Davis, 2825 50th Street, Sacramento, CA 95817, USA Fragile X syndrome (FXS), a genetic disorder resulting from a mutation of the FMR1 gene, is the most common inherited cause of intellectual disability (Mazzocco 2000). Recent epidemiological studies report a prevalence of approximately 1 in 2,500 individuals for the full mutation (FernandezCarvajal et al. 2009; Hagerman 2008; Pesso et al. 2000). In FXS, the mutation results in a reduction or absence of FMRP, the protein produced by the FMR1 gene. This reduction in FMRP levels is thought to account for a range of physical, behavioral, and neuropsychological features and deficits observed in affected individuals (Loesch et al. 2004). Males with FXS typically exhibit a more severe phenotype than females, characterized by moderate to severe intellectual disability (Loesch et al. 2004; Skinner et al. 2005). Among females, cognitive functioning ranges from within the average 336 range to moderate intellectual disability (Cronister et al. 1991; de Vries et al. 1996). FXS also affects social development, and behaviors characteristic of autism are frequently observed. The percentage of individuals with FXS meeting the diagnostic criteria for autism has been estimated at 25–50%, with virtually all males with FXS displaying at least some behaviors characteristic of autism (Philofsky et al. 2004; Demark et al. 2003; Lewis et al. 2006; Kaufmann et al. 2004). In addition to cognitive and social impairments, substantial delays in language are characteristic of the FXS phenotype (Sudhalter et al. 1991; Murphy and Abbeduto 2003). Individuals with FXS demonstrate delays in receptive and expressive language that span multiple domains. Impairments may be evident in vocabulary size and complexity, morphosyntax (knowledge of the formation and structure of words), and syntax (application of principles for constructing sentences; see Finestack et al. 2009). Deficits in social aspects of language (e.g., maintenance of a conversation topic, appropriate turn-taking) and frequent use of repetitive language are also common (Belser and Sudhalter 2001; Murphy and Abbeduto 2007; Roberts et al. 2007; Mazzocco et al. 2006). Although considerable research has documented a characteristic profile of difficulties with acquisition and use of language among groups of individuals with FXS relative to their chronological-age peers, it is also important to acknowledge the high degree of within-syndrome variability in language outcomes (Abbeduto et al. 2007). For example, children and adolescents with FXS exhibit striking variation in expressive skills, with some individuals limited to nonverbal communication and others (mostly females) exhibiting age-appropriate competence with most language mechanics (Murphy and Abbeduto 2003; Mazzocco et al. 2006). It has been speculated that differences among individuals in language functioning arise from a combination of biological influences, neurocognitive and behavioral characteristics, and variations in the linguistic and social environment (Finestack et al. 2009). Identifying key predictors of linguistic outcomes in FXS may be useful for a number of reasons. First, given the prevalence of language impairments in this population and the profound impact communication deficits can have on quality of life and educational attainment (e.g., Van Agt et al. 2011; Durkin et al. 2009), developing a better understanding of sources of language variation in FXS is of considerable public health importance. Such knowledge could aid in identifying early markers of language impairment and in counseling families regarding prognosis and treatment recommendations. Second, knowledge of the underlying factors that predict language outcomes could be beneficial for constructing more effective interventions for language problems in affected individuals. Finally, a better understanding of the longitudinal course and influences on language development in this population may have relevance not only for individuals with FXS, but also for the J Neurodevelop Disord (2011) 3:335–347 identification and treatment of language difficulties within the general population and other neurodevelopmental disorders. Unfortunately, there have been few attempts to empi (...truncated)