Quantifying age-related changes in mirror overflow in children and adolescents with attention-deficit/hyperactivity disorder.

Attention‐deficit/hyperactivity disorder (ADHD), characterized by persistent patterns of inattention, hyperactivity, and impulsivity, is the most common neurodevelopmental disorder. In addition to well‐established cognitive deficits, children with ADHD demonstrate motor atypicalities, including motor overflow, defined as overt or covert involuntary movement that occurs during a voluntary movement. Specifically, excessive overflow, unintentional movements in homologous muscles that mimic intentional movements on the opposite side of the body, has been frequently observed in children with ADHD as compared with their typically developing (TD) peers—particularly in childhood., , Excessive motor overflow has primarily been linked with hyperactivity in children with and without ADHD,, although one study found motor overflow to be associated with inattention. Additionally, excessive overflow is thought to be related to other manifestations of inhibitory control, a core deficit of ADHD, such as impaired selective attention, divided attention, and response inhibition,, , and is further evidenced by associations with deficits in neural circuits underlying inhibitory control., ADHD‐related motor overflow deficits had been studied extensively using measures, in which overflow is simply marked as “present” or “absent” (i.e., via the Physical and Neurological Examination for Subtle Signs [PANESS]). Addressing limitations of this somewhat qualitative approach, we developed a finger‐transducer‐based method to more precisely quantify mirror overflow during sequential finger tapping, revealing excessive overflow in pre‐pubertal children with ADHD relative to TD children., , Although a singular quantitative study of youth ages 8–17 found excessive motor overflow resolves in adolescence in youth with ADHD, there have been no studies of adolescents with ADHD using a quantitative method, so the degree to which ADHD‐associated overflow resolves into adolescence remains unclear. Addressing this gap, the current study aimed to quantitatively assess age‐related changes in mirror overflow within the context of a unilateral sequential finger‐tapping task in youth ages 8–18 years with and without ADHD. The increased resolution of a quantitative approach provides greater sensitivity to detect the relationship between overflow and ADHD‐related deficits, enabling us to confirm if excessive mirror overflow in children with ADHD resolves by adolescence, as implied by prior qualitative studies. Additionally, the current quantitative approach may be applied to other developmental and medical conditions associated with motor abnormalities for a more sensitive measure of overflow trajectory. Given prior qualitative studies showing mirror overflow decreases with age in youth with ADHD,, we hypothesized that both youth with ADHD and TD youth would show age‐related decreases in mirror overflow, but the effect would be greater in youth with ADHD, such that diagnostic differences observed would be less robust in adolescence. Given well‐established recognition of earlier maturation of motor control in the dominant hand than in the nondominant hand, and in conjunction with reports of individuals showing more overflow when engaging in voluntary movement of the nondominant hand than of their dominant hand,, , we evaluated mirror overflow in the left and right hand, separately and together. We predicted that mirror overflow in the left and right hand would show differential age‐related changes, with the effect of age being greater during voluntary movement of the left hand. Finally, we predicted that mirror overflow would be positively correlated with ADHD symptom severity.