Abstract
Question Effects of transcranial direct current stimulation (tDCS) vary in cognitive domains. Whereas anodal tDCS mostly showed the expected excitatory effect known from the motor cortex, cathodal tDCS often failed to hinder performance. It was suggested that the time of the task relative to tDCS may play a critical role for the effects. We aimed to investigate the effects of anodal and cathodal tDCS on semantic processing. Furthermore, the task was passed either during (online) or following (offline) stimulation to explore the relationship between tDCS polarity and time of the task. Methods Three groups (20 subjects each) passed a lexical decision task (words vs. pseudowords) after being stimulated with either anodal, cathodal or sham tDCS (offline groups). Three other groups are designed to pass the task during stimulation (online groups). The active electrode was placed over CP5 and the reference electrode extracephalic on the left shoulder. Stimulation intensity was 1 mA. For the offline groups, tDCS was applied for 15 min. For the online groups, stimulation lasted for the whole time of the task (25 min). Results For the offline groups, mean reaction times (RTs) on words of the cathodal group were significantly decreased with respect to the sham group (mean cathodal group: 584.8 ms, mean sham group: 636.5 ms, p = 0.04). Additionally, a trend towards decreased RTs in the anodal group compared to the sham group was observed (mean anodal group: 597.1 ms, p = 0.05). There was no difference in RTs on words between the anodal and the cathodal group ( p = 0.55). Up to now, 13 subjects participated in the online conditions (10 anodal, 3 sham, cathodal not yet measured). First visual inspection of the data suggests that the online anodal group might show faster RTs than the offline anodal group. Conclusions Our preliminary data support the view that effects of tDCS on motor cortex excitability cannot be transferred to other cortical regions. Additionally, there is evidence that tDCS effects depend on task timing.
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