Fig 1.
Simulated electric field map (E_n) on adult human brain’s gray matter surface induced by HD-tDCS.
(a) Left inferior frontal gyrus stimulation in Experiment 1. The central electrode is at F7, and the four return electrodes are at F5, AF7, FT7, and F9 electrode positions (10–20 EEG System). Peak electric field strength of 0.21 V/m (red) at the pars opercularis and pars triangularis. (b) Left dorsolateral prefrontal cortex stimulation in Experiment 2. The central electrode is at F3, and the four return electrodes are at AF3, F1, F5, and FC3 electrode positions (10–20 EEG System). Peak electric field strength of 0.26 V/m (red) at Brodmann’s areas 9 and 46.
Fig 2.
A trial starts with the appearance of the fixation cross for 0.5 secs. The first premise appears after the fixation cross disappears, and the second premise appears 10 seconds after the first one. The conclusion is then shown 5 seconds after the second premise. For deductive reasoning (left), participants had to indicate whether the conclusion was valid (right button) or invalid (left button). For inductive reasoning (right), participants had to indicate whether the conclusion was strong (right button) or weak (left button). Task adapted from Goel and Dolan (2004) [14].
Fig 3.
Reaction times for deductive (categorical and propositional) and inductive (propositional) arguments before and after left inferior frontal gyrus tDCS.
(a) RTs for easy trials. Anodal stimulation significantly increased the RTs for categorical and propositional deductive arguments. The post-stimulation RTs were significantly longer for categorical deductive arguments after anodal than cathodal stimulation. (b) RTs for difficult trials. There were no significant stimulation-specific effects on RTs. pre = before stimulation, post = after stimulation, * = indicates significant differences before and after stimulation, + = indicates significant differences between stimulation conditions. Pairwise comparisons were Bonferroni corrected with a significance level of p ≤ 0.05. Data are presented as mean ± standard error of mean.
Fig 4.
Accuracy rates for deductive (categorical and propositional) and inductive (propositional) arguments before and after left inferior frontal gyrus tDCS.
(a) ARs for easy trials. Anodal stimulation significantly reduced the ARs for propositional inductive arguments. The post-anodal stimulation ARs for propositional inductive arguments were significantly lower than after sham and cathodal stimulations. (b) ARs for difficult trials. ARs significantly increased and decreased for categorical deductive and propositional inductive arguments after sham stimulation, respectively. For categorical and propositional deductive arguments, the pre-stimulation ARs in the cathodal condition were significantly higher than in sham and anodal conditions. pre = before stimulation, post = after stimulation, * = indicates significant differences before and after stimulation, + = indicates significant differences in ARs between stimulation conditions. Pairwise comparisons were Bonferroni corrected with a significance level of p ≤ 0.05. Data are presented as mean ± standard error of mean.
Fig 5.
Reaction times for deductive and inductive arguments before and after tDCS stimulation of the left dorsolateral prefrontal cortex.
(a) RTs for easy categorical and propositional deductive and inductive arguments. (b) RTs for difficult categorical and propositional deductive and inductive arguments. RTs for categorical inductive arguments significantly decreased after anodal and cathodal stimulations. The post-stimulation RTs for categorical inductive arguments were shorter in anodal and cathodal conditions than in sham. pre = before stimulation, post = after stimulation, * = indicates significant differences before and after stimulation, + = indicates significant differences in RTs between stimulation conditions. Pairwise comparisons were Bonferroni corrected with a significance level of p < 0.05. Data are presented as mean ± standard error of mean.
Fig 6.
Accuracy rates for deductive and inductive arguments before and after tDCS of the left dorsolateral prefrontal cortex.
(a) ARs for easy categorical and propositional, deductive and inductive arguments. (b) ARs for difficult categorical and propositional, deductive and inductive arguments. ARs for categorical inductive arguments significantly increased after anodal and cathodal stimulation. The post-anodal stimulation ARs for categorical inductive arguments were significantly lower in the anodal than in the cathodal condition. The ARs significantly decreased after anodal and cathodal stimulation for propositional inductive arguments. The post-cathodal stimulation AR was significantly lower than those in the sham and anodal conditions. pre = before stimulation, post = after stimulation, * = indicates significant differences before and after stimulation, + = indicates significant differences in RTs between stimulation conditions. Pairwise comparisons were Bonferroni corrected with a significance level of p ≤ 0.05. Data are presented as mean ± standard error of mean.