Targeting Interhemispheric Balance to Modulate Language Processing: a tDCS Study in Healthy Volunteers

Transcranial direct current stimulation (tDCS) is a non-invasive brain stimulation method that has a high potential to enhance the effect of behavioral aphasia treatment. A large body of research shows that inhibitory stimulation of the right hemisphere is beneficial but there are fewer studies where it is combined with excitatory stimulation of the left hemisphere in a bilateral montage (the combination of anodal stimulation of the left hemisphere and cathodal stimulation of the right hemisphere). Moreover, no study has yet compared bilateral stimulation to both necessary control conditions (separate stimulation of the left and the right hemispheres). Finally, only Giustolisi et al. (2018) used a sentence-level task to test the effects of tDCS. This study investigates the effects of bilateral tDCS in an experiment with both of the control conditions and with both sentence-level and word-level tasks. 72 healthy right-handed Russian speakers (aged 18-32) were tested. The participants completed two linguistic tasks (lexical decision and sentence comprehension). Each participant was assigned to one of the three groups, receiving anodal stimulation of the left Broca’s area, cathodal stimulation of its right-hemisphere homologue, or bilateral stimulation (combination of the two); the participants also received placebo stimulation on a different day. The participants practiced the tasks during stimulation and were tested immediately after it. No significant effect of stimulation, i.e., improvement in accuracy and/or reaction times in either task, compared to sham, was found. Moreover, there were no significant interactions between stimulation (real versus sham) and experimental group (bilateral versus left anodal versus right cathodal). These findings will help gain new insights into the effects of tDCS in healthy population. However, a follow-up study is necessary to test whether the same tDCS settings might still be effective in patients with aphasia.