37 / 218
35
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.