Effects of transcranial direct current stimulation of left and right inferior frontal gyrus on creative divergent thinking are moderated by changes in inhibition control

Abstract

Divergent thinking (DT) as one component of creativity is the ability to search for multiple solutions to a single problem and is reliably tested with the Alternative Uses Task (AUT). DT depends on activity in the inferior frontal gyrus (IFG), a prefrontal region that has also been associated with inhibitory control (IC). Experimentally manipulating IC through transcranial direct current stimulation (tDCS) led to alterations in DT. Here, we aimed at further examining such potential mediating effects of IC on DT (measured as flexibility, fluency, and originality in the AUT) by modulating IC tDCS. Participants received either cathodal tDCS (c-tDCS) of the left IFG coupled with anodal tDCS (a-tDCS) of the right IFG (L-R + ; N = 19), or the opposite treatment (L + R-; N = 21). We hypothesized that L + R- stimulation would enhance IC assessed with the Go NoGo task (GNGT), and that facilitated IC would result in lower creativity scores. The reversed stimulation arrangement (i.e., L- R +) should result in higher creativity scores. We found that tDCS only affected the originality component of the AUT but not flexibility or fluency. We also found no effects on IC, and thus, the mediation effect of IC could not be confirmed. However, we observed a moderation effect: inhibition of left and facilitation of right IFG (L-R +) resulted in enhanced flexibility and originality scores, only when IC performance was also improved. We conclude that inducing a right-to-left gradient in IFG activity by tDCS is efficient in enhancing DT, but only under conditions where tDCS is sufficient to alter IC performance as well.

Keywords: AUT (alternative uses task); Creativity; Divergent thinking (DT); Flexibility; Fluency; IFG; Originality; tDCS.

Fig. 1

Schematic-view illustrates the experimental design. Participants’ creativity and inhibitory control (IC) were accessed by the Alternative Uses Task (AUT) and Go NoGo task (GNGT) before and after tDCS of the inferior frontal gyrus (IFG). During the AUT, participants were asked to write down creative ideas about alternative uses of two objects: A brick and a paper clip within 2 min for each object. Different words were used for the AUT in the pre- and post-test. The order of these words was counterbalanced. The task measured fluency (number of different types of categories of ideas), originality (uniqueness/novelty of the ideas), and flexibility (number of switches between different ideas). Weak direct current (1 mA) was applied between two 4 × 6 cm² large wet sponge electrodes placed over F7, and F8 according to the international 10–20 EEG system for 30 min. In a randomized order, half of the participants received anodal tDCS of the right IFG and cathodal tDCS of the left IFG, whereas the other half received the opposite current flow

Fig. 2

A schematic graph illustrating the modeling of current flow when applying 1.0 mA tDCS for F7 anodal (right-hand side) and P8 cathodal (left-hand-sided) stimulation. Red color points to the inward (anodal) electrical field (EF), while blue represents outward (cathodal) EF. The middle graph refers to the 2D electrode layout montage of a 338-point head model while the lateral graphs besides the head montage show EF magnitude plots. The color bar indicates the field intensity of tDCS stimulation. This model stimulation had been created using Soterix Medical High-Definition transcranial Direct Current Stimulation (HD-tDCS)

Fig. 3

Creativity scores (y axes) achieved by the participants during the AUT task after tDCS of left and right IFG. L − R + relates to anodal stimulation of the right inferior frontal gyrus (IFG) (coupled with cathodal stimulation of the left IFG) while L + R − refers to anodal stimulation of the left IFG (coupled with cathodal stimulation of the right IFG). Panels (a–c) refer to the scores for fluency, originality, and flexibility, respectively. Shown are estimated marginal means and SE at a fixed level of the respective pre-value as a covariate in the ANCOVA. There was a significant effect of tDCS condition on originality, only. Cf., Table 1, for detailed statistics)

Fig. 4

a, b show dprime values and reaction times (RTs), respectively, for the two conditions of the GO NOGO task (i.e., 50:50 and 90:10) pre and post tDCS. L − R + relates to anodal stimulation of the right inferior frontal gyrus (IFG) (coupled with cathodal stimulation of the left IFG) while L + R- refers to anodal stimulation of the left IFG (coupled with cathodal stimulation of the right IFG). Shown are estimated marginal means and SE at a fixed level of the respective pre-value as a covariate in the ANCOVA. Cf., Table 2 for detailed statistics

Fig. 5

Association between changes in dPrime (d’delta90; 90/10 condition of GNGT) and originality (a) and flexibility (b) scores (estimated marginal means) after tDCS. Linear regression lines with 95% confidence intervals are displayed. L − R + relates to anodal stimulation of the right inferior frontal gyrus (IFG) (coupled with cathodal stimulation of the left IFG) while L + R − refers to anodal stimulation of the left IFG (coupled with cathodal stimulation of the right IFG). ANCOVA expressed a significant interaction effect of d’delta90 and type of tDCS on post tDCS creativity scores for originality (F (1,19) = 5.521, p = 0.030, pEta² = 0.225) and flexibility (F (1,19) = 9.901, p = 0.005, pEta² = 0.343): at positive levels of d’delta90 (increased GNGT performance after tDCS), creativity scores were higher after L − R + stimulation than after L + R − stimulation. No difference between stimulation conditions was revealed when there was no change or a decline in GNGT (negative d ‘delta90 values; cf. Table 3 for detailed statistics)