Proefschrift_Holstein

General introduction

processing during the execution of the rewarded task-switching paradigm ( box 2.3 ). To this end, I compared a group of adults who were diagnosed with ADHD with a group of subjects without ADHD while they performed the rewarded task switching paradigm in a functional MRI environment ( box 2.4 ). Effects of dopamine in these patients were assessed by testing the patients both after their normal dose of methylphenidate (i.e. after Ritalin®, or an equivalent dose of Ritalin for those usually taking Concerta®; box 2.2b ) and after withdrawal from their normal medication ( box 2.2b ). This enabled me to assess whether ADHD medication affects the integration of motivation and cognitive control signals. Further, to account for inter- individual differences in task performance and neural processing during this task (Aarts et al., 2010), individual variability in the dopamine transporter genotype was taken into account ( box 2.2c ). The results revealed that ADHD was accompanied by excessive signalling in the striatum when patients had not taken their medication. However this effect depended on the DAT1 genotype, and was only present in a subset of patients. In addition, this excessive striatal response was normalized after intake of methylphenidate. Surprisingly however, we did not replicate the previous observation that motivated cognitive control varies according to the DAT1 genotype in healthy subjects ((Aarts et al., 2010) and chapter 3 ). One major difference between these studies was the age of the subjects: those in chapter 3 were ~22 years old, whereas the control group in chapter 4 was on average 38 years old. In chapter 5 I therefore aimed to assess whether the age of the participants could indeed explain the differences between these studies.

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