Proefschrift_Holstein

Controlling dorsolateral striatal function via anterior frontal cortex stimulation

Figure 7.4 Main effects across sessions A: Left: The main effect of Reward anticipation (high > low; red); middle: Task switching (switch > repeat; green) and right: Response switching (switch > repeat; blue) at a threshold of t = 3.14 (PUNC < 0.001). Circles with black outline indicate the stimulation sites for the anterior prefrontal cortex (left), dorsolateral prefrontal cortex (middle) and premotor cortex (right). The rendered images show regions with a search depth of 4mm. B: For illustration purposes and to visualize the relative location of the main effects, the figure depicts the overlay of the main effects of Reward anticipation (red), Task switching (green), and Response switching (blue) at a low threshold (t > 1.28, p < 0.1) for coronal slices (top row: anterior to posterior) and axial slices (bottom row: ventral to dorsal). procedure (Ashburner and Friston, 2005). This procedure resulted in a set of inverse parameters allowing the conversion of the stimulation targets in MNI space to subject space. TMS During the experimental sessions, a cTBS protocol was employed that was the same as that reported by Huang and colleagues (2005). These authors applied cTBS at 80% of the aMT and reported a depression of MEP amplitudes over a subsequent period up to 60 minutes (see also Volman et al., 2011; Wischnewski and Schutter, 2015). TMS pulses (biphasic) were administered through a figure-eight coil (75mm diameter), connected to a MagPro X100 stimulator (Mag Venture, Denmark). Mean MNI coordinates for the three cortical stimulation sites were projected onto each individual’s structural scan using a frameless stereotactic neuronavigation system (Localite, Sankt Augustin, Germany). We used standard electromyogram (EMG) recordings to visualize MEPs from the first dorsal interosseous (FDI) muscle of the right hand and to determine the resting MT (rMT), using a standard protocol (Schutter and van Honk, 2006; Volman et al., 2011). During the determination of the aMT, participants rested their right hand on a pillow while squeezing a small roll of tape with a pincer grip at 20% of their maximum strength, contracting their FDI muscle continuously. The aMT was defined as the lowest stimulation intensity over the contralateral motor cortex that elicited reproducible MEPs (in at least 5 out of 10 successive stimulations). The aMT was 24%-37% (mean 30.44%, SD 3.61) of the maximum stimulator output. During the experimental sessions, a cTBS protocol (bursts of three 50 Hz pulses every 200 ms for 40s, i.e. a total of 600 pulses) was administered at 80% of the aMT. The TMS coil was placed as close to the target location (i.e. the aPFC, dlPFC or PMC) as possible using the Localite software. During the intake session, participants received the exact same protocol for 10 instead of 40 seconds over each of the three stimulation sites to acquaint them with the sensation of cTBS.

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