Neurocognitive Domains

Neurocognitive function was operationalized by two domains: working memory and processing speed. The specific tasks selected were those that have empirical support as areas of deficit in youth with ADHD [18, 46] and are important for achievement in math [47]. Tasks were included as individual predictors rather than indices due to the established need for a greater understanding of the respective contributions of working memory components from individual tasks [29]. Age-corrected standard scores were calculated, per WISC-IV scoring rules.

Working memory was assessed with the two subtests of the Wechsler Intelligence Scale for Children—Fourth Edition (WISC-IV; [48]), and one supplementary measure from the WISC-IV Integrated Battery (Spatial Span Backward; [48]). In the Digit Span Backward subtest, children are required to repeat 2–9 digits backwards. In Letter-Number Sequencing, children reorder a string of alternating numbers and letters into letters in alphabetical order and numbers in numeric order. In Spatial Span Backward, the child first watches the experimenter touch a series of blocks in different spatial locations and then responds by touching the reverse pattern of spatial locations, with an increasing number of blocks in each trial. As in other investigations, in this study Digit Span Backward and Letter-Number Sequencing were used to assess auditory-verbal working memory, while the Spatial Span Backward subtest was used to assess visuospatial working memory (e.g., [16, 18, 20, 24]). Scaled scores were used for each subtest.

Processing speed was assessed with the WISC-IV Coding subtest scaled scores, in which a key is presented to the child that pairs numbers and symbols. The child was then given 2 min to write the corresponding symbol below 90 numbers presented in a grid. Coding measures visual-motor processing speed and complexity, as well as motor coordination and working memory.

Abbreviated Full-Scale IQ (FSIQ) was used as a covariate in analyses and, as in other studies assessing cognitive functioning in youth, was calculated from the Vocabulary and Matrix Reasoning subtests of the Wechsler Abbreviated Scale of Intelligence (WASI; [43]), with standard scores calculated. A brief estimate of FSIQ was included to control for variability in intellectual functioning characterized by only the domains less susceptible to the presence of neurodevelopmental disorders (i.e., verbal comprehension and perceptual reasoning; [19]).