Anne Fleur Kortekaas-Rijlaarsdam

107 REINFORCEMENT LEARNING IN ADHD: EFFECTS OF MPH 5 Generalization of learning Fifty-eight TD children (94%) met the predefined criteria for successful acquisition of the stimulus-response relationship and therefore entered the test phase. In the ADHD group, 55 children (87%) met the criteria to enter the test phase when treated with placebo and 57 children (92%) met these criteria when treated with MPH. Children who did not meet criteria for entry in the test phase were equally distributed across our ADHD and TD group ( χ 2 (1)=.07, p =.790) as well as across treatment groups ( χ 2 (1)=.04, p =.842). Effect of group on generalization of knowledge There was a main effect of group ( F (111)=5.29, p =.023, d =.42) indicating that children with ADHD were less able to generalize acquired knowledge to novel stimulus combinations as they showed larger difference scores between the last block of the learning phase and the test phase. Effect of MPH treatment on generalization of knowledge The main effect of treatment on the ability to generalize acquired knowledge to other stimulus pairs just escaped conventional levels of significance ( F (46.5)=3.07, p =.086, suggesting that MPH may affect generalization of knowledge, with small to medium effect size ( d =.30). Generalization of knowledge did not differ between children with ADHD who were treated with MPH and controls ( F (113)=.28, p =.598), suggesting that methylphenidate normalized performance levels to that of controls. Reversal Learning Effect of group on reversal learning There were no effects of group or pair on reversal learning ( F (153.5)=1.01, p =.316 and F (111)=1.05, p =.307, respectively). The interaction between group and pair was not significant ( F (111)=.44, p =.507). Effect of treatment on reversal learning The interaction between treatment and pair was significant ( F (152.4)=3.93, p =.049. Although post hoc analyses revealed no effect of MPH on the individual pairs ( F (51.7)=1.2, p =.277 and F (55.2)=2.3, p =.135 for AB and CD pairs respectively), Table 5.2 shows that accuracy increases with almost 5% for the AB pair in the MPH condition, whereas accuracy decreases with almost 7% for the CD pair in this condition.