Participants

Fifty-four healthy adults (mean age 21.8 ± 1.9 years; 29 women) voluntarily participated in this study. The a priori definition of appropriate sample sizes was based on effect sizes reported in the relevant literature using tDCS for similar group comparisons19,23. Participants were evenly and randomly assigned to three experimental groups (n = 18 in each group) according to the type and the timing of brain stimulation delivery during the task, namely the TrainStim group (a-tDCS during initial training), the ShamStim group (sham-tDCS during initial training), and the IncubStim group (a-tDCS during incubation). All were right-handed, as assessed by the Edinburgh Handedness Inventory24. Prior history of drug or alcohol abuse, neurological, musculoskeletal, psychiatric or sleep disorders, constituted exclusion criteria. This study was approved by the Research Ethics Committee of the Center of Research and Innovation in Sport (University Claude Bernard Lyon 1). All participants signed an informed consent form in agreement with the terms of the Declaration of Helsinki (2013). The experiment was performed in accordance with relevant guidelines and regulations. Participants were not aware of the aims and hypotheses of the study.

Experimental procedure

We used the well-known NRT, which advantageously provides an objective and reproducible assessment of both insight and analytical solving strategies (see detailed methodology in Debarnot et al.4). This task is constituted by a series of eight numbers that must be sequentially combined following initial rules to obtain a final numeric solution (Fig. 1). Importantly, participants were not aware that all strings contain the same underlying pattern with the last three responses mirroring the three preceding ones. This pattern allows the second response to be identical to the final one, and offers (when discovered) an efficient strategy to reach the solution from the very early steps in the series. Thus, participants may either apply the initial rules step-by-step (i.e. analytic strategy), or explicitly discover the hidden rule, by change in the representation of the NRT, which was reflected by a collapse into response times to enter the correct solution (i.e. insight strategy).

Figure 1 Number Reduction Task (NRT). Each stimulus string was always composed by digits ‘1’, ‘4’ and ‘9’. Participants were asked to find the final response (R7) corresponding to the solution. To do so, participants might perform sequential processing of digits pairwise from the left to the right according to two simple rules: the ‘same rule’, which states that the result of two identical digits result in the same digit (response 1 here); the ‘different rule’, which states that the result of two non-identical digits is the remaining third digit (responses 2 and 3 here). Critically, the mirror pattern present in the stimulus string (1 4 1 4 1 here) is reflected in the pattern of response string such as responses 2–4 always mirror responses 5–7, hence the second digit response is systematically the final solution. Participants who gain insight of this hidden rule through change in the NRT representation show an abrupt and significant decrease in response time to give the correct solution (R7), and in doing so press systematically the space bare at a significant earlier step (R2). By contrast, analytical participants proceed through the problem step-by-step using the same and different rules toward the solution. Full size image

Before the experiment, all participants were equipped with the same tDCS montage, but parameters and timing of stimulation delivery were different (Fig. 2). Standardized NRT instructions were given by the experimenter, and included a short practice block of five trials to ensure the correct understanding of the initial rules and goal of the task. Then, all participants performed an initial training session on the NRT during 3 blocks, which was followed by an incubation period whereby participants were asked to remain quietly seated and relaxed during 20 min. Finally, they performed a retest session that included 10 blocks of 30 trials each. Importantly, whenever the experimenter noticed a short cut to find the correct solution number in using systematically the space bare earlier at each trial (i.e. the second response number), an additional block of practice was performed to verify the representational change from analytical to insight solving strategy. Right after the retest, two questions were systematically asked to participants: “how did you proceed during the task”, and “what did you do exactly”. This debriefing was used to subjectively probe whether participants adopted either the insight or the analytical strategy.

Figure 2 Experimental design. Concomitant a-tDCS was applied over the lPPC either during training (three blocks) or the incubation period. Excitatory stimulation at 2 mA during 19 min was applied in the TrainStim and the IncubStim groups, while false stimulation (0 mA during 19 min) was applied in the ShamStim group. No further stimulation was performed during retest (10 blocks). Finally, during the debriefing, participants were asked to explain how they proceeded during the task. Full size image

Transcranial direct current stimulation

All participants were equipped with tDCS (STARSTIM, Neuroelectrics) that included two saline-soaked sponge electrodes, and a small anodal electrode (25 cm2, current density 0.08 mA/cm2) to elicit a more focal stimulation and larger cathodal electrode (35 cm2) in order to avoid meaningful stimulation of the reference site. Theoretical simulation of the current density magnitude is illustrated by Fig. 3. The anode was localized over the lPPC (P3 based on 10–20 EEG system), while the cathode was placed over the right supraorbital region, referred to as Fp2. Hence, the anodal stimulation was concomitantly administered during the initial training or incubation period in the TrainStim and IncubStim groups, respectively. In both groups, the current was ramped up to reach 2 mA during the first 30 sec, and remained at this intensity for 19 min, and then reduced to 0 mA during the last 30 sec. For a high level of blinding, sham stimulation was delivered during initial training in the ShamStim group, and presented similar 30 sec up and down current modalities, but remained at 0 mA during 19 min.

Figure 3 Simulation of the current density magnitude (V/m) generated by 2 mA a-tDCS to the lPPC (StimWeaver, Neuroelectrics, Spain). Left: sagittal view of the left hemisphere, Middle: superior view, Right: sagittal view of the right hemisphere. Full size image

Data analysis