Anodal transcranial direct current stimulation of the right anterior temporal lobe did not significantly affect verbal insight

Abstract

Humans often utilize past experience to solve difficult problems. However, if past experience is insufficient to solve a problem, solvers may reach an impasse. Insight can be valuable for breaking an impasse, enabling the reinterpretation or re-representation of a problem. Previous studies using between-subjects designs have revealed a causal relationship between the anterior temporal lobes (ATLs) and non-verbal insight, by enhancing the right ATL while inhibiting the left ATL using transcranial direct current stimulation (tDCS). In addition, neuroimaging studies have reported a correlation between right ATL activity and verbal insight. Based on these findings, we hypothesized that the right ATL is causally related to both non-verbal and verbal insight. To test this hypothesis, we conducted an experiment with 66 subjects using a within-subjects design, which typically has greater statistical power than a between-subjects design. Subjects participated in tDCS experiments across 2 days, in which they solved both non-verbal and verbal insight problems under active or sham stimulation conditions. To dissociate the effects of right ATL stimulation from those of left ATL stimulation, we used two montage types; anodal tDCS of the right ATL together with cathodal tDCS of the left ATL (stimulating both ATLs) and anodal tDCS of the right ATL with cathodal tDCS of the left cheek (stimulating only the right ATL). The montage used was counterbalanced across subjects. Statistical analyses revealed that, regardless of the montage type, there were no significant differences between the active and sham conditions for either verbal or non-verbal insight, although the finding for non-verbal insight was inconclusive because of a lack of statistical power. These results failed to support previous findings suggesting that the right ATL is the central locus of insight.

Fig 1. Experimental procedure and tDCS electrode montages.

(A) A schematic representation of the experimental procedure. The shaded area represents the time of tDCS application. Each subject completed all three sessions on separate days; the first tDCS session was conducted 1–4 days after the learning session, and the two tDCS sessions were separated by an interval of 1 week (more than 1 week for a few subjects). The order of active and sham tDCS sessions was counterbalanced across subjects to reduce the possibility of any potential learning effects. In tDCS sessions, the testing phase started 5 min after tDCS onset to ensure that there was sufficient change in cortical excitability [10]. (B) tDCS electrode montages. Left, the bi-cephalic electrode montage. Right, the extra-cephalic reference electrode montage. The anodal and cathodal electrodes are shown in red and blue, respectively. These figures were drawn using the COMETS toolbox.

Fig 2. An example of the non-verbal insight problems used in the matchstick arithmetic task.

Column one displays the problem, and column two gives the corresponding solution. The last column indicates the problem type. The magenta lines in columns one and two indicate the sites where manipulation occurred. Type-A insight problems were used in the learning session and practice phase of the tDCS session. Type-B and Type-C problems were used in the testing phase of the tDCS sessions.

Fig 3. An example of the verbal insight problem (the RAT task).

Column one displays the problem. Column two gives the corresponding solution. English translations of each two-word phrase are as follows: 異郷 (strange land), 口癖 (habitual saying), 序盤 (initial phase), 異論 (different opinion), 口論 (dispute), 序論 (introduction).

Fig 4. Results of the matchstick arithmetic task during the learning session.

The left and right panels show the proportion of correct answers and response times for correct answers, respectively. Each gray column represents the mean (across subjects) value for each block. Error bars represent standard deviations. Asterisks indicate significant (p < 0.05) differences between blocks. (A) The “bi-cephalic” group. (B) The “extra-cephalic” group.

Fig 5. Results of the two-digit addition arithmetic task during the learning session.

The left, middle, and right panels show results for the number of correct answers, proportion of correct answers and response time for correct answers, respectively. Each gray column represents the mean (across subjects) value for each block. Error bars represent standard deviations. Asterisks indicate significant (p < 0.05) differences between blocks. (A) The “bi-cephalic” group. (B) The “extra-cephalic” group.

Fig 6. Results of all tasks during the test phase of the tDCS sessions.

The left and right panels show the number and proportion of correct answers, respectively. The dark and light gray columns represent the effect sizes for the “bi-cephalic” and “extra-cephalic” montages, respectively. The error bars represent 95% confidence intervals.