Figure 1.
Schematic illustration of the experimental protocol.
Figure 2.
Determination of the parietal electrode positions and setup for the tDCS application.
A: To validate whether positions P3 and P4 of the 10–20 international system for EEG electrode placement correspond to the bilateral IPS, external stereotactic fiducial markers were taped to the scalp of 4 participants at P3 and P4 before entering the scanner. B: Panel B reflects the anatomical validation of the correct position of the fiducial markers over the IPS. C: Panel C depicts functional validation of the correct position of the markers by displaying the original fMRI effects for target identity and distractor distance of the study by Klein and colleagues (2009) in the IPS on each of the individual normalized MRI scans. The blue crosshairs mark the centers of the fiducials. As can be seen, the electrode positions P3 and P4 correspond very closely to the localization of the activation peaks in the IPS observed by Klein et al. (2009). D: Setup of the tDCS application and presentation of the numerical task. The application of tDCS was transferred by two square scalp electrodes covered with conductive rubber (5×5 cm each) and red saline-soaked synthetic sponges over the target sites (P3 and P4) and two big reference electrodes (10×10 cm each, cf. the blue cables in Panel D) in the supra-orbital region.
Figure 3.
Schematic illustration of the tDCS set-up.
As current flow through the.electrodes was regulated by two independent constant drain channels this system is linear. The DC-STIMULATOR MC provides freely programmable, micro-processor-controlled constant current sources using independent channels. By continuous (hardware- and software-based) monitoring of the electrode impedance it was ensured that the current path remains at the correct value for each hemisphere independently.
Figure 4.
Photograph of the experimental tDCS montage used in the current study.
The active small electrodes (5×5 cm2) are shown in red, the large reference electrodes in blue. Please note that the distance between active electrodes and reference electrodes is larger than 5 cm as recommended to minimize current flow through the skin (see [66] for a review).
Figure 5.
Results of the computer simulation of the experimental set-up as used in the present study.
Please note that the resulting field intensity did not exceed 0.01 V/m in frontal brain areas but reached values equal or larger than 0.23 V/m in the bilateral intraparietal corices. According to Nitsche and Paulus [50] a minimum field intensity of 0.017 V/m is necessary to modify cortical excitability by tDCS in humans.
Table 1.
Mean response latencies for the three stimulation conditions in ms separately for the different distractor distance×target identity combinations.
Figure 6.
Effect of distractor distance and Stroop congruity separated for stimulation condition.
Panel A reflects differences in RT between small and large distractor distances in the two digit addition task for the different stimulation conditions. The significant difference of the distractor distance effect between cathodal and anodal stimulation is marked by an asterisk. Panel B depicts RT differences between incongruent and congruent items in the colour word Stroop control task for the different stimulation conditions. Error bars reflect standard errors of the mean (SEM).