Modulating prefrontal cortex activity to alleviate stress-induced working memory deficits: A transcranial direct current (tDCS) study

Fig. 1. Behavioral data of WM task performance for accuracy and d-prime are shown. a.) Violin plots for the number of hits (accuracy) for all three n-back blocks are shown here. The highest possible number of hits in the task was 31. The y-axis shows the hit scores and the x-axis shows different tDCS groups (stimulation conditions) separated by emotional conditions (control, stress) colored violet and blue respectively. The median value is marked by horizontal red lines and quartiles by black lines for each violin plot. Significant differences are marked by asterisks (*) in black. b.) Violin plots for the d-Prime values for all three n-back blocks are shown here and plotted as described for accuracy.

Fig. 2. Physiological alterations induced by stress before, during, and after combined WM performance and tDCS. a.) Heart rate before, during, and after task performance. Significant differences of HR between conditions (stress, control) were observed only in the sham condition, with increased HR in the stress as compared to the control condition for all time points, as shown by a red horizontal line marked by an asterisk (*). The y-axis denotes HR changes from baseline (first time point of the experiment as described in the methods section), and the x-axis denotes each minute of the experiment (from the end of the stress induction procedure) (t1-t23). Resting-state (RS) and n-back task time frames are highlighted in the background with grey and orange colored watermarks (orange represents the resting state and grey the n-back task). b.) Cortisol levels before and after task performance are plotted. The y-axis displays mean cortisol levels (nm/l), and the x-axis shows the time course relative to the start of the movie clips. Error bars denote ± SEM. We found significant differences of cortisol concentration between conditions (stress, control) only in the sham condition for all time…

Fig. 3. EEG results for the 3-Back load condition. a.) averaged ERP for all groups and conditions for EEG channel Cz. The x-axis represents time (in ms), and the y-axis shows the amplitude (µV). The time range of interest of the P200 (120–200 ms) component is highlighted by a grey box. Respective averaged amplitude values were used for the statistical analysis and shown as a bar plot next to it. The bar plot shows the averaged P200 component values for all groups and conditions, with the x-axis indicating the groups and the y-axis the potential amplitude (µV). Significant differences are marked by an asterisk (*) in black color. b.) averaged ERP for all groups and conditions for the EEG channel Pz and the P300 (200–400 ms) component amplitude, depicted as in a.). c.) averaged task-based alpha activity from parietal channels (Pz, P1, and P2) for all groups and conditions, the x-axis indicates time (ms), and the y-axis power (in dB). The time range of interest for ERD (200–600 ms) is marked by a grey box, and the mean power value within this time frame is plotted in a bar graph with the x-axis indicating groups, and y-axis power (dB). Significant differences are marked by an asterisk (*)…

Fig. 4. Heat map for Pearson's correlation values in the 3-back load WM task for the EEG (alpha ERD, theta ERS, P200 amplitude, P300 amplitude) and other physiological indexes (cortisol levels before and after task performance, HR, and HRV during task performance) with the performance parameters Hits and d-prime for all stimulation and emotional conditions. The colors in the plot are showing r values with the color bar ranging from 0.5 to −0.5, red showing positive and blue showing negative correlations. For significant correlations, r-values, and p-values of the associations are shown.

Fig. 5. Simulated distribution of the electric field in the brain based on computational modeling for the selected electrode montage to target the (a) vmPFC with the electrodes over Nz (Nasion), F7, F8, Ex19, and Ex20, and (b) left DLPFC, with the electrodes over F3, Fp1, Fz, C3, and F7. The current amplitude was set to 2 mA, and all electrodes were discs with a 1 cm radius.

Fig. 6. Experimental procedures: The participants attended three one-day experimental sessions where the first session included completing questionnaires, undergoing a medical check, and practicing the WM task. The next two sessions included the main experimental part – involving the control and stress-inducing video clips - which were administered with a washout period of 7 days. For the experimental days (sessions 2 and 3), after the arrival of the participants, an EEG cap was prepared, and the stress induction part was done as described in detail in previous work (Roy et al., 2024) with randomized session order (control and stress condition). After resting state EEG (RS1), the movie clip presentation and another resting state EEG (RS2), participants performed the n-back task with three blocks (2, 3, and 4 back) in randomized order. The 3-back task is shown as an example, where the correct response (hit) is highlighted by a respective arrow. tDCS using small electrodes with a 1 cm radius was given in accordance with the stimulation group (sham, anodal dlPFC, and anodal vmPFC tDCS). After task performance, another RS EEG with 2 min Eyes Open (EO) and 2 min Eyes Closed (EC) conditions…