Anne Fleur Kortekaas-Rijlaarsdam

141 SUMMARY AND DISCUSSION 7 In contrast to our predictions, MPH did not improve the cognitive or motivational functioning of children with ADHD in our sample. This was in spite of the fact that children performed less well on these measures, compared to TD controls. These findings oppose previous findings on MPH-related improvements in cognition (Coghill et al., 2014; Pietrzak et al., 2006) and motivation (Shiels et al., 2009). Although our cognitive tasks were comparable in design as task used in previous studies, literature on the effect of MPH on explicit (questionnaire) measures of academic motivation is lacking so far. It is unlikely that trial duration was too short to result in cognitive and motivational improvements, as previous studies on the effects of MPH on cognition and motivation have similar, or shorter, durations (Coghill, Seth, et al., 2014; Pietrzak et al., 2006; Shiels et al., 2009). Perhaps MPH-related improvements in cognition and motivation have explanatory value only in case of (more) severe deficits in cognition and motivation. This conclusion appears valid as our lack of findings is not due to a ceiling effect or lack of power and we investigated an elaborate set of cognitive and motivational measures often affected in children with ADHD (Carlson, Booth, Shin, & Canu, 2002; Luman, Oosterlaan, & Sergeant, 2005; Mullane, Corkum, Klein, & McLaughlin, 2009; Scholtens, Rydell, & Yang-Wallentin, 2013; Willcutt, Doyle, Nigg, Faraone, & Pennington, 2005) which are related to academic underperformance (Biederman et al., 2004; DiPerna et al., 2005; Guay et al., 2003; Gut et al., 2012; Mayes & Calhoun, 2007b; Preston et al., 2009; Thorell, 2007; Volpe et al., 2006). Therefore, our results down play the importance of these cognitive and motivational variables in the explanation of MPH-related academic improvements. We do acknowledge, however, that there are other cognitive and motivational measures that may be relevant here and that may show MPH-related improvements essential for academic performance. For example, children with ADHD show deficits in response inhibition (Faraone et al., 2015; Willcutt et al., 2005) and previous literature suggests that these deficits partly mediate the impact of ADHD diagnosis on academic (under)performance. As there is evidence for positive effects of MPH on response inhibition (Coghill et al., 2014), these improvements may mediate MPH-effects on academic performance. In addition, we found a moderating role for academic abilities which implies that MPH-treatment is most effective for children with below-average mathematical abilities. This finding is promising as it is for these children improvements in math performance are most relevant. Also, if future research confirms these findings, it may be useful to take academic performance into account to predict academic outcomes. Taken together is important to emphasize that effects of MPH on academic performance were small which attenuates the importance of the model behind these limited