Experimental design
GROUP B, CORTEX
A randomized, sham-controlled, double-blind, crossover study design was implemented. Participants came for 6 experimental sessions (see Fig. Experimental design. Each participant is assessed at 6 time points, T1 corresponds to an EEG screening, a screening questionnaire and informed consent, T2 to the optional fMRI, and T3-T6 to the randomized interventions (offline/online, tDCS/sham).Participants were randomly divided into 2 groups: participants in Group A were stimulated in the left Dorsolateral Prefrontal Cortex (lDLPFC) and performed the N-Back Task in the concurrent task condition, whereas participants in Group B were stimulated in the right Inferior Frontal Gyrus (rIFG) and performed the Flanker task in the concurrent task condition. In each stimulation session, both groups randomly received either a sham or an active tDCS stimulation. Each tDCS stimulation session unfolded as follows: once participants were equipped with the stimulation headset, resting state EEG was recorded during 2 min eyes open (EO) and 2 min eyes closed (EC), followed by 20 min of tDCS stimulation. In the concurrent task condition, the task started after 2.5 min of tDCS stimulation and ended 2.5 min before the end of the stimulation. In the non-concurrent task condition, participants were instructed to sit and relax with eyes opened while receiving tDCS. For all participants, after stimulation, EEG data were recorded during a second resting state period (2 min EO, 2 min EC) and then during the execution of the following cognitive tasks: Flanker Task, N-Back Task and Continuous Performance Task (Fig. T3 to T6 trial design. During each trial, 2 min eyes open and 2 min eyes closed resting state EEG are being recorded before and after the tDCS stimulation. Participants then came back 4 times to receive 4 different tDCS interventions (T3–T6) in a randomized order: active tDCS with a concurrent task, active tDCS with a non-concurrent task, sham tDCS with a concurrent task and sham tDCS with a non-concurrent task. After each tDCS intervention, 3 tasks were included in the experimental design for behavioral effect evaluation. These 3 tasks were always performed after the stimulation session had ended, and independently of the stimulation condition (active vs. sham, concurrent vs. non-concurrent task). Each stimulation session lasted 20 min. For the concurrent type of intervention, the task started and ended 2.5 min after and before the stimulation respectively (in order to avoid current ramping distractions). The tasks performed after the stimulation had a duration of approximately 16 min for Flanker Task and N-Back N-Back Task, and approximately 18 min for CPT.