Fig. 1: Paradigm and tDCS modeling.

A Schematic illustration of the task. Participants are instructed to react as quickly to colored squares in one of four positions denoted by crosses that remained on throughout the paradigm by pressing on a spatially and chromatically corresponding button on a keyboard. B Task structure. The task used a 5-element repeat sequence that was modeled after previous studies [21, 25, 26, 104]. We used four different SRTT sequences (3, 1, 4, 2, 4), (2, 3, 1, 2, 4), (1, 3, 4, 2, 3), (4, 2, 1, 3, 2), pseudo randomly assigned to one of the four stimulation conditions i.e., Sham, Motor-anodal, Motor-cathodal and Visual-cathodal, per individual, such that no one received the same sequence twice. Two blocks of SRTT, 12 min each, were administered during tDCS/EEG. C tDCS Field Strength Mapping. Pad placements for the Motor-cathodal and Motor-anodal conditions followed the M1-SO (left primary motor-right supraorbital) scalp positions used in prior tDCS SRTT studies. For Visual cortex stimulation, the anode pad was placed over the vertex (Cz) and the cathode pad was placed on the scalp area (POz) overlaying the cortical dorsal visual area [76]. For sham stimulation, the pads were placed in the same positions as for motor stimulation; however, the stimulator only delivered 30 seconds of ramp up and down. The montage resulted in predominant current flow in premotor and somatomotor regions during motor cortex stimulation and dorsal visual and superior parietal regions during visual cortex stimulation [25].