Proefschrift_Broeders

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Controlling dorsolateral striatal function via anterior frontal cortex stimulation

Figure 7.4

Main effects across sessions

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.

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.