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Chapter 7
the aPFC modulates neural responses in the striatum, but it did not induce any behavioral
changes, which is not uncommon with offline TMS (van Schouwenburg et al., 2012; Tupak et
al., 2013). However, the absence of a behavioral effect precludes us frommaking any claims as
to whether stimulation of the aPFC and the subsequent effects on the striatum had beneficial
or detrimental effects on functional processes. Finally, contrary to our expectations, there
was no modulation of task-related processing after stimulation of the dlPFC or PMC. The
dlPFC region we stimulated showed significant overlap with the main effect of task switching.
Nevertheless, stimulation of the dlPFC site failed to alter neural processing as a function
of task switching or as a function of the interaction between task switching and response
switching. One explanation is that the region we stimulated is not crucial for task-related
processing in corticostriatal circuits in our paradigm. In fact, a meta-analysis has suggested a
role for a more posterior region of the PFC in task switching (i.e. the inferior frontal junction;
x, y, z coordinates: -40, 4, 30) (Derrfuss et al., 2005), which overlaps with the peak in the PFC
activated by our task-switching contrast, suggesting indeed that the region we stimulated was
too dorsal to target the corticostriatal circuitry involved in task switching. The absence of
an effect of PMC stimulation likely reflects the finding that the network activated during
Response switching did not show any overlap with the stimulation site in the PMC. We may
have therefore failed to stimulate the region involved in response switching, and as a result we
did not observe task-related modulation of PMC stimulation in the striatum. The SNS account
discussed above predicts that the spiraling SNS connections are organized in an ascending
way. We set out to test this idea by showing that stimulation of the aPFC, but not of the dlPFC
or PMC, would affect processing in the striatum as a function of reward. This is exactly what
we observed, although due to the absence of any effects after stimulation of the dlPFC and
PMC we cannot be confident that stimulation of the dlPFC and PMC was effective. Thus, the
results clearly show that task-related integration can occur across corticostriatal circuits and
that is occurs in a unidirectional manner, from anterior/ventral to posterior/dorsal parts of
the striatum. However, we cannot rule out that stimulation of the dlPFC or PMC, had it been
effective, could also modulate activity in more anterior/ventral parts of the striatum.
The current study is the first to show functional interactions between corticostriatal circuits
during the integration of task-related goals, by causally manipulating neuronal excitability.
The results of this TMS study show that corticostriatal circuits communicate in order to
facilitate the translation of information across goals or functional domains. Understanding
exactly how cognitive goals and subsequent actions are informed by reward motivation is
important when understanding the etiology of a number of neuropsychiatric disorders with
deficits in integrating between these signals and/or deficits in corticostriatal circuits (for
a review see Shepherd, 2013), such as attention deficit hyperactivity disorder (Aarts et al.,
2015; Hong et al., 2015), schizophrenia (Morris et al., 2015), obsessive compulsive disorder
(Graybiel and Rauch, 2000), and addiction (Belin and Everitt, 2008; Tang et al., 2015).