Dual tasks (DTs) require additional control processes that temporally coordinate the processing of the two component tasks. Previous studies employing imaging as well as noninvasive stimulation techniques have demonstrated that the dorsolateral prefrontal cortex (dlPFC) is causally involved in these task-order coordination processes. However, in these studies, participants were instructed to match their processing order to an externally provided and mandatory order criterion during DT processing. Hence, it is still unknown whether the dlPFC is also recruited for rather voluntary order control processes, which are required in situations that allow for intentional and internally generated order choices. To address this issue, in two experiments, we applied anodal (Experiment 1) and cathodal (Experiment 2) transcranial direct current stimulation during a random-order DT in which participants could freely decide about their order of task processing. In our results, we found facilitatory and inhibitory effects on voluntary task-order coordination because of anodal and cathodal transcranial direct current stimulation, respectively. This was indicated by shorter RTs when participants intentionally switched the task order relative to the preceding trial during anodal as well as a reduced tendency to switch the task order relative to the preceding trial during cathodal stimulation compared with the sham stimulation. Overall, these findings indicate that the dlPFC is also causally involved in voluntary task-order coordination processes. In particular, we argue that the dlPFC is recruited for intentionally updating and implementing task-order information that is necessary for scheduling the processing of two temporally overlapping tasks.

Dual-task scenarios require a coordinated regulation of the processing order of component tasks in light of capacity limitations during response selection. A number of behavioral and neuroimaging findings suggest a distinct set of control processes involved in preparing this task order. In this study, we investigated electrophysiological correlates of task-order preparation in a variant of the overlapping dual-task paradigm with cue-determined task order that resulted in trials with blockwise fixed task order as well as trials with repeated and switched task order in blocks with variable task order. During the cue–stimulus interval, we found an earlier centroparietal order-mixing positivity and a later parietal order-switch positivity. A decoding approach based on multivariate pattern analysis showed that the order-mixing positivity is a necessary prerequisite for successful order selection, whereas the order-switch positivity appears to facilitate the implementation of a new task order after its selection. These correlates of order preparation share striking similarities to commonly found potentials involved in the preparation of individual tasks in the (single-)task-switching paradigm, which is strong empirical support for the account that the underlying preparatory processes are to be considered as higher-level control signals that are implemented independently of specific task representations.

Dual tasks are characterized by the requirement for additional task-order coordination processes that schedule the processing order of two temporally overlapping tasks. Preliminary evidence from functional imaging studies suggests that lateral pFC (lPFC) activation correlates with implementing these task-order coordination processes. However, so far, it is unclear whether the lPFC is also causally involved in coordinating task order during dual-task performance and which exact mechanisms are implemented by this brain region. In this study, we addressed these open issues by applying online TMS during a dual-task situation. For this purpose, participants performed a dual task in fixed-order blocks with a constant order of tasks and in random-order block, in which the order of tasks varied randomly and thus demands on task-order coordination were increased. In Experiment 1, TMS of the lPFC compared with control TMS conditions impaired dual-task performance in random-order blocks, whereas performance in fixed-order blocks was unaffected by TMS. In Experiment 2, we tested for the specificity of the lPFC TMS effect on task-order coordination by applying TMS over the preSMA. We showed that preSMA TMS did not affect dual-task performance, neither in fixed-order nor in random-order blocks. Results of this study indicate that the lPFC, but not the preSMA, is causally involved in implementing task-order coordination processes in dual-task situations.