Language, Reading, and Resting-state Oscillatory Power in ADHD, DLD, and Comorbid ADHD/DLD
Abstract
High rates of comorbidity in attention-deficit/hyperactivity disorder (ADHD) and developmental language disorder (DLD) have motivated interest in utilizing psycholinguistic and physiological metrics to distinguish between these conditions. However, past studies have focused on one disorder and overlooked the significant overlap in symptoms between ADHD and DLD. Consequently, less is known about how these assessments can distinguish between “pure” (no comorbidities) cases of either disorder or comorbidities. This thesis aims to elucidate the basis of these overlapping disorders by examining language, reading, and resting-state oscillatory power differences and assessing their potential in distinguishing ADHD and/or DLD. Chapter 2 presents a scoping review of research on language abilities in ADHD and DLD. It revealed that children with ADHD had better morphosyntax/grammar, general/core language, receptive, and expressive language than those with DLD. However, there were variations in assessments of phonological processing, syntax, narrative language, and vocabulary. On the other hand, performance on semantic, pragmatics, and figurative language assessments were similar between groups. Standardization across studies is highlighted as crucial to consolidate inconsistencies and gain a clear understanding of the distinct language difficulties associated with each disorder. Chapter 3 builds on the findings of Chapter 2 by investigating whether the presence of a comorbid language disorder exacerbates language and reading difficulties in ADHD. Additionally, this chapter explores the ability of psycholinguistic assessments to distinguish between groups: ADHD (combined or inattentive subtype), DLD, and comorbid ADHD + DLD. Measures of reading efficiency could distinguish between the two types of ADHD, but not between other groups. Interestingly, scores on the standard language screener were no worse for children with ADHD + DLD than children with DLD only. These findings offer valuable insights into differential diagnosis and the identification of comorbidity In Chapter 4, resting-state electroencephalography (EEG) was used to examine oscillatory power differences in ADHD, comorbid ADHD + DLD, and control groups. It also examined whether groups could be distinguished based on their oscillatory power patterns. While EEG power spectra differences were observed between pure and comorbid ADHD + DLD, resting-state EEG was unable to accurately distinguish any of the groups with high accuracy, suggesting limited reliability as a diagnostic tool. Chapter 5 summarizes the findings of this thesis in relation to the shared cognitive deficits in ADHD and DLD and pathways contributing to comorbidity.