Proefschrift_Holstein

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 ).

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