Discussion
The present study investigated whether modulation of excitability in the left-lateralized executive control network, as modulated by a prefrontal tDCS setup with anode over left dlPFC, facilitated the executive control function of prioritized task shielding in dual tasking. Results demonstrated that atDCS, as compared to sham tDCS, reduced crosstalk between the two tasks indicating improved task shielding. Importantly, improved task shielding was observed in conditions of highest dual-task demands, i.e., the largest temporal overlap between the two tasks.More precisely, in the condition of sham tDCS, crosstalk was stronger the more simultaneously both tasks were being processed. The close temporal proximity of stimuli at high temporal task overlap allows for more simultaneous task-component processing, which typically increases crosstalk between tasks. With less temporal task overlap, crosstalk typically declines. Thus, our findings in the sham tDCS condition mirror the usual result pattern obtained in dual-task research without any brain stimulation (e.g.,The findings of the present interventional approach, therefore, extend previous imaging studies suggesting a significant role of the dlPFC in task shielding (e.g.,Based on the present observations, we can only speculate how the modulation of the underlying executive control network by atDCS over left dlPFC facilitates processes of task shielding. One way of interpreting our results would be that atDCS could have aided conflict resolution by biasing the signals needed for Task 1 prioritized processing. Recent neuroimaging studies, for example, have shown a conflict-related attentional biasing in favor of task-relevant stimulus dimensions in single tasksAlternatively, tDCS-induced facilitation of task shielding may be directly related to enhanced processing efficiency rather than conflict resolution. That is, neural accounts to explain training effects on cognitive task performance postulate that training can lead to more efficient processing of the already engaged neural circuitsSuch an assumed benefit in neural efficiency, however, did not show up in the present RT measures, as the effects of improved task shielding under atDCS were found in error rates only. A selective impact of tDCS on error rates is not uncommon and has been reported frequentlyIn contrast to the error rates, RT results showed an increase of RT1 with longer SOAs. In dual-task paradigms with mostly short SOAs, participants tend to execute both responses at the same time, which leads to a typical strategic Task 1 response deferment with increasing SOAWhile the present results demonstrate that atDCS over the dlPFC improves task shielding in conditions of high between-task interference, further research combining functional neuroimaging and brain stimulation techniques may be informative to which extent facilitated task shielding is a consequence of tDCS-modulated activity and/or functional connectivity in underlying brain networks. In addition, the present effects of tDCS on task shielding were assessed across all experimental blocks, i.e., during and immediately after stimulation