Proefschrift_Holstein

Controlling dorsolateral striatal function via anterior frontal cortex stimulation

each trial was either “correct” or “incorrect”. During the intake session and on arrival each experimental day, participants completed a second practice block that was exactly the same as the task used in the actual paradigm, only shorter (i.e. 24 trials). Finally, a block (32 trials) without reward or feedback was administered in the scanner right before the actual task (as well as during the intake session). The average response times on four trial types (arrow/word * task-switch/task-repeat) were used to determine each individual’s response window. These response deadlines were used to account for inter-individual and inter-session differences in response speed and subsequent task difficulty. The paradigm consisted of 160 trials and lasted ~35 minutes with a 30s break every 32 trials. In the breaks and at the end of each run (i.e. after 160 trials) the cumulative amount of money the participant earned was displayed on the screen, (max. €12.80). Participants were informed in advance that we would keep track of the total amount of money on each of the six runs and that their earnings on one run would be added to their financial compensation as a bonus. At the end of the final experimental session, the participant rolled a dice to determine which run’s earnings was added as a bonus. Behavioral analysis Behavioral analyses were performed on the response times (RTs) and error rates. The first trial of each block, trials with extremely fast responses (<100ms) and trials on which participants failed to respond were excluded from analyses. In addition, trials on which the response was incorrect were excluded from RT analyses. Results were analyzed using a repeated measures ANOVAwith the factors TMS condition (stimulation or baseline), Reward (high or low), Task- Switching (switch or repeat) and Response Switching (switch or repeat) for each stimulation site (aPFC, dlPFC, PMC). We transformed the response times (log) and proportions of error (arcsine(√x)) to improve the distribution of the data (4 out of 48 RT variables: Shapiro-Wilk p < 0.023; 14 out of 48 error rate variable: Shapiro-Wilk p < 0.039). The stimulation sites for the motivation, cognition and action network were determined by assessing the peak activations in the frontal cortex in an independent study using the same paradigm (Aarts et al., 2010 unpublished observations). A region in the anterior PFC (aPFC; -30, 60, 8, Brodmann area 10, figure 7.4a red circle) was identified as part of the reward network (high reward cue > low reward cue); a region in the dlPFC (-36,36, 20, Brodmann area 46, Figure 7.4a green circle) was identified as part of the cognitive network (task switch > task repeat); and a region in the PMC (-28, 10, 66, Brodmann area 6, 7.4a , blue circle) was identified as part of the action network (response switch > response repeat). Each participant’s structural scan was coregistered to the standard SPM8 T1 template (Montréal Neurological Institute; MNI) and segmented using a unified segmentation TMS procedure Stimulation sites

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