The current study sought to examine performance of children with ADHD versus typically developing controls across the tasks of the WISC-IV-I Processing Speed Index, dissociating the executive “processing” components necessary to response selection and preparation from the initial perceptual and terminal graphomotor output demand of PS. We also sought to explore the contribution of these elements to reading fluency. In this well screened sample of children without basic word reading or language difficulties, there were no differences between ADHD and control groups in basic graphomotor copying speed (i.e., Coding Copy); whereas, there were significant differences in the aspects involving response selection (i.e., CodingR), with the ADHD group slower than controls. As a group, children with ADHD were also significantly less efficient on measures of oral contextual and non-contextual reading fluency and silent non-contextual reading fluency. Further, this non-graphomotor element “processing speed” (i.e., that involving response selection) was significantly associated with oral reading fluency performance. Importantly, these measures of fluency required not just rapid performance, but both efficient and accurate responding, thus including a substantial executive demand in response selection and preparation. Among constructs hypothesized to play a role in this efficient responding, verbal attention span as well as the executive control elements of verbal and spatial working memory accounted for a significant proportion of additional unique variance in children’s performance on Coding, over and above that accounted for by sex, Coding Copy, ADHD-related symptom severity, and GAI. In contrast, measures of reaction time, rapid naming speed, inhibition, and intra-individual response variability did not contribute. Notably, measures of verbal and spatial working memory accounted for unique variance in Coding, even after controlling for the respective maintain demand (e.g., verbal span/spatial span). Working memory (in these analyses, assessed primarily by the WISC-IV-I Letter-Number Sequencing and Spatial Span Backwards tasks) may play an important role in performance of this task due to the effortful nature of cognitive control required to complete the tasks, the additional “step” of controlled processing required in re-ordering or transforming information mentally, or its more novel presentation. Each of these characteristics highlights the executive control involved, which may help to explain the contribution to the response selection component of PS.
Taken together, these results suggest that the “cognitive” (executive) and graphomotor output components of PS can be dissociated, with these cognitivecomponents contributing uniquely to slowed output speed in ADHD (Chhabildas, 2001
; Hinshaw, 2002
; Rucklidge & Tannock, 2002
; Willcutt, 2005
). Given previous findings of skeletomotor and oculomotor slowing in children with ADHD (Cole, et al., 2008
; Mahone, et al., 2009
), one might expect that ADHD-related deficits on the WISC-IV-I PSI are due to largely to the motor control demands of the tasks. Our findings, suggest, however, that difficulty with response selection may be uniquely responsible for at least part of these observed findings. The Coding task does present a greater vertical scanning demand than the Coding Copy task so there may also be differences in perceptual demand which contribute to different group performance on these tasks; however, the reading fluency measures demand horizontal scanning rather than vertical scanning/tracking, so it is unlikely that this perceptual difference alone contributes to group differences on reading fluency measures. The observed differences between groups on a residualized measure of Coding, controlling for sex and Coding Copy performance, suggest that differences may exist between groups at the level of response selection (i.e., CodingR
), with this difference in response selection potentially contributing to differences in apparent response efficiency. To our knowledge, this is the first study to examine this dissociation between components of the WISC-IV-I Coding in children with ADHD.
The observed differences between children with ADHD and controls in more frontally-mediated executive components of processing speed also contributed to differences in performance on measures of reading fluency, even in this highly screened sample. Given recent findings that children with ADHD, even without comorbid reading or language disorders, still show weaker reading fluency than controls (Ghelani, et al., 2004
; Willcutt, et al., 2007
), and our findings that verbal span and working memory predict reading fluency for both children with and without ADHD, these results support the idea that executive control skills, especially working memory, play an important role in development of reading fluency, beyond word reading accuracy (e.g., Willcutt, et al., 2005
) for all children, and thus are of particular importance in later elementary school years and beyond. These data are consistent with recent work suggesting that impairments in specific components of executive control contribute to deficits in higher-level aspects of reading skills, even for children with intact decoding skills (Locasio et al., in press
; Sesma et al., 2009
Results implicating executive aspects of processing speed, including working memory, provide new evidence for involvement of skills dependent on more anterior (premotor, prefrontal) circuits, as well as on long-range frontal-posterior connections involving these areas, in children with ADHD. These behavioral findings are consistent with recent anatomic MRI evidence (Shaw, 2009
; Shaw, 2008
; Shaw, 2007
) implicating anomalous frontal lobe development in children with ADHD, including decreased volumes of supplementary motor complex (SMC) and prefrontal (dorsolateral, medial) cortex volumes, especially among boys with ADHD (Mahone, Richardson, et al., 2009
). The SMC, in particular the rostral portion or “pre-SMA”, is thought to be critical to response control and selection (Mostofsky & Simmonds, 2008
), and may contribute to reduced efficiency of response speed, whereas anomalous prefrontal development may drive the working memory contributions to ADHD-related behavioral inefficiency.
Limitations and Future Directions
As a function of the high level of screening in the present study, participants were of average to above average intellectual ability, with control participants tending to show higher than average GAI (with less variability than the normative sample). This may reflect a potential sampling bias with regard to parental willingness to involve their children in research, either due to an interest in potentially identifying areas of concern in children with ADHD or in identifying superior cognitive ability in control participants. On a related note, given the attempt to statistically adjust for this difference in mean GAI score between groups, our results may actually represent a low estimate of the effects of executive aspects of “processing” on children’s reading fluency. Due to the overlap in essential elements between conventional measures of IQ and executive function, especially involving working memory and response preparation/processing speed, which were of particular interest in this paper, statistical “control” of IQ has been suggested to produce anomalous results when examining group differences in executive functioning (Dennis et al., 2009
). Furthermore, a recent meta-analysis suggests that reduced IQ scores in the ADHD population relative to typically developing controls may be specifically related to attentional dysregulation and poor test-taking behavior rather than deficits in “intelligence,” per se (Jepsen, Fegerlund & Mortensen, 2009
). It is important to note that children with ADHD in this study were tested off medication, which Jepsen and colleagues (2009)
also suggest is likely to result in lower IQ scores. Furthermore, as some data suggest the relation of children’s executive function to intellectual ability may differ at the ends of the IQ distribution (Delis, 2007
; McGee, 2009
), the observed relation of working memory to PS and reading fluency measures may be different in children with lower levels of verbal ability. The current sample was not large enough to test for differences in this association across levels of ability.
In addition, the recent evidence from structural neuroimaging (Mahone et al., 2009
) suggests that boys and girls with ADHD may share a common anomaly in premotor circuit (especially SMC) development; whereas in boys (but not girls) with ADHD (especially in this age range), the anomalous development of prefrontal circuits may persist throughout childhood (Qiu et al., 2009
). Thus, the observed association between “processing”, verbal working memory, and reading fluency may differ across sexes. Future research with larger and more equally distributed samples of boys and girls will be required to address that issue.
The current study used PS measures from the WISC-IV-I, which was designed to allow for a more careful examination (e.g., “process analysis”) of the component skills involved in completion of the more traditional WISC tasks. Development of additional measures permitting the dissociation of response components will be helpful in further examining differences in aspects of PS, including motor or verbal output and the executive components of response selection and preparation. Furthermore, it will be important to investigate the association between the slowed “processing” typical of children with ADHD and reading comprehension skills, as reading comprehension is believed to rely even more explicitly on efficient “processing” of material and verbal working memory skills.