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Chapter 7
the targeted sites were involved in the task. To this end, we generated a region of interest
by drawing a sphere of 8mm around the stimulation coordinate (
figure 7.4a
) and assessed
effects of Reward in the aPFC site, effects of Task switching in the dlPFC site and effects of
Response switching in the PMC site, using small volume corrections (SVC). In addition, we
assessed the effects of Reward on Task switching, the effects of Task switching on Response
switching and the three-way interaction between Reward, Task switching and Response
switching across all sessions.
Next, we assessed whether stimulation of the aPFC, relative to baseline, (i.e. the contrast
aPFCSTIM-BASE), changed Reward-related processing, the effect of Reward on Task
switching and the interaction between Reward, Task switching and Response switching. For
dlPFC stimulation, we assessed whether stimulation of the dlPFC, relative to baseline (i.e.
the contrast dlPFCSTIM-BASE), changed the effect of Task switching and the interaction
between Task switching and Response switching. Finally, we assessed whether stimulation
of the PMC compared with baseline (i.e. the contrast PMCSTIM-BASE) altered Response
switching.
Effects that survive a family wise error (FWE) correction (peak PFWE < 0.05) were considered
as significant. We assessed effects at the whole-brain level, but with specific hypotheses
regarding the striatum. Therefore we applied small volume corrections in the caudate nucleus
or putamen: effects in the caudate nucleus were assessed for the integration of Reward, Task
switching and the interaction between Reward and Task switching, but effects in the putamen
were assessed for the main effect of Response switching and any interactions with Response
switching (i.e. Reward x Task switching x Response switching and Task switching x Response
switching). For any significant (PSVC_FWE < 0.05) effect in either of these regions (e.g. the
caudate nucleus), we assessed regional specificity by testing for the same effect in the other
region of the striatum (e.g. the putamen).
For visualization purposes, statistical maps are overlaid onto a study-specific template
(see
preprocessing of task-related fMRI data
) displayed both at a low threshold to allow
assessment of physiological plausibility (t = 1.28, p < 0.1) and at more stringent thresholds to
determine statistical significance (e.g. t = 3.14, p < 0.001;
figure 7.4 and 7.5
).