Background: tDCS of Broca’s region has mostly been used for investigating speech production (Fiori et al., 2013; Marangolo et al., 2013), while the putative role of this area in semantic processing, such as processing concrete vs. abstract words, is still poorly understood. This study aimed to assess the impact of tDCS of Broca’s area on abstract and concrete word acquisition.

Methods: Twenty novel word forms and twenty novel meanings (ten abstract and ten concrete) were created. Three groups of participants (N=24 each) received 15 minutes of anodal/cathodal/sham (placebo) tDCS over Broca’s area before the learning session. Each novel word was presented in context of 5 sentences, by reading which participants could gradually deduce its meaning. Free Recall, Recognition, Free-form Definition and Multiple-choice Semantic Judgment (SJT) tasks were implemented to assess learning outcomes both immediately after learning and with a 24-hour delay, to test consolidation effects. Mann-Whitney pairwise comparison and Wilcoxon signed-rank test (FDR-corrected) were used for statistical analysis of accuracies and reaction times.

Results: In the Free Recall task, cathodal group was more successful than sham group when recalling concrete words on Day 1 (p=0.036). Also on Day 1, participants recognized novel abstract words more accurately after anodal and cathodal tDCS of Broca's area in comparison with the sham condition (p=0.044).

On Day 2 participants in all groups recalled fewer words of both types than on Day 1 (all p-values≤0.018). Accuracy of recognition of novel words was lower on Day 2 vs. Day 1 in the cathodal (p=0.020 for concrete words) and the anodal groups (p=0.060 for concrete and 0.016 for abstract words). Definition quality estimates were lower on Day 2 than Day 1 for all groups and both semantics types (all p-values≤0.012). In the SJT, the sham group demonstrated accuracy decrease for abstract words (p=0.020), whereas the cathodal group exhibited it for concrete ones (p=0.026). Such a decrease, however, was observed for neither abstract nor concrete words in the anodal group.

Conclusions: In sum, both anodal and cathodal tDCS of Broca’s area improved contextual acquisition of novel words. At the same time, anodal tDCS seemed to ensure better retention of new words after a 24-hour delay. Furthermore, the results indicate a relatively greater involvement of Broca’s area in acquiring abstract semantics.