Handwriting deficits in the comorbidity of dyslexia and attention-deficit/hyperactivity disorder and their electrophysiological correlates.

Handwriting is a fundamental skill crucial for effective communication and learning. While previous research has demonstrated handwriting impairments in individuals with attention-deficit/hyperactivity disorder (ADHD) and developmental dyslexia (DD) separately, the impact of their comorbidity on children's handwriting abilities remains largely unexplored. This study employed a written production task and a copying task to measure handwriting speed and legibility in children with DD-only (N = 37), ADHD-only (N = 39), comorbid DD and ADHD (COM, N = 44), and typically developing children (N = 36). The findings revealed that the COM group exhibited significantly diminished handwriting speed and legibility compared to the typically developing group. Furthermore, the COM group demonstrated handwriting speed comparable to the DD-only and ADHD-only groups but reduced legibility compared to the DD-only group, suggesting both similarities and additive effects of handwriting impairments in the COM group. Finally, we examined the association between handwriting performance and intrinsic neural activity using closed-eye electrophysiological (EEG) recordings in the COM group. The results revealed that the handwriting deficit in the COM group was linked to neural activity in the frontal regions, suggesting the involvement of executive functions and/or motor execution in impaired handwriting. Collectively, this study uncovers severe handwriting impairments in individuals with comorbid DD and ADHD, underscoring the importance of assessing handwriting skills for accurate diagnosis and effective treatment of comorbid conditions.