Lateralization of activity in the parietal cortex predicts the effectiveness of bilateral transcranial direct current stimulation on performance of a mental calculation task.

Transcranial direct current stimulation (tDCS) is a non-invasive technique that moderates cognitive and motor function. The effects of tDCS on cognitive and motor tasks vary among individuals. However, the source of the inter-individual variability remains unknown. The purpose of the present study was to examine whether the effect of bilateral tDCS on the performance of mental calculations differs among individuals according to the functional lateralization of parietal activity observed during a mental calculation task. Sixteen healthy subjects (11 males and five females, aged 20-23 years) participated. Laterality of parietal activity during a mental calculation task was evaluated using functional magnetic resonance imaging. Subjects also performed the mental calculation task pre-, during-, 30 min post-, and 60 min post-tDCS. Bilateral tDCS with the anode over the left parietal cortex and the cathode over the right parietal cortex shortened response times of the mental calculation task in subjects with left-hemispheric parietal lateralization, but not in subjects with bilateral parietal activation. This indicates that inter-individual variability in laterality of brain activity might be an important factor underlying the effect of bilateral tDCS. In conclusion, bilateral tDCS over the parietal cortex enhanced the performance of mental calculations in subjects with left-hemispheric parietal lateralization.