Hierarchical control in task switching
Keywords: hierarchy
meta-control
Hierarchical control
EEG
Task switching

Human flexible behaviour is often seen in everyday life tasks. These tasks (e.g., making coffee) are composed of actions (e.g., pouring sugar) that are typically nested within a large task structures made of superordinate components and nested subcomponents. Task switching has been adopted widely as an effective tool to explore the mechanisms of flexible behaviour and can be easily adapted to real-life situations. The core hypothesis explored in this thesis was that there might be another level of control that is responsible for coordinating and scheduling actions in task switching, which I label "meta-control".
My first project aimed to establish robust behavioural evidence of meta-control. To test this hypothesis, switching efficiency was treated as a measure of meta-control influence. In a novel design, participants’ beliefs about switch probability were manipulated through explicit instruction, allowing manipulation of meta-level control independent of the specific sequence of tasks required. The first three behavioural experiments demonstrated behaviorally that instructions influenced the efficiency of switching even when low-level (bottom-up) experience was matched, and that this effect was motivation-dependent. In follow-up studies, this meta-control influence was found to bias voluntary task selection. Two EEG studies aimed to characterize the level at which instruction affected processing. Motor and task levels were examined with lateralized motor potentials and oscillatory alpha power, respectively. Effects of instruction only existed on alpha power. Overall, the results suggest that expectancy prompts the adoption of distinct control modes across sequences of trials.
The second project explored meta-control in a task switching paradigm with a hierarchical task structure in very short (2-4 trial) sequences that were preceded by instructions about switch frequency or switch position. The experiments focused on the benefits and costs of sequence structure, based on the hypothesis that trial sequences are treated as coherent units. Three behavioural studies were conducted for testing this hypothesis. The first utilized instructions about switch frequency to test for sequence transition effects and their influence on switching performance at the trial level. In two subsequent experiments, instructions made explicit the position of a task switch. The results confirmed that instructions about switch position helped participants to build useful sequence representations, and that alternating between sequences influenced trial-level switch processes. Generally, sequence representations have a persisting influence across trials and a pronounced impact at the first trial position of sequences.
The experiments in this thesis provide evidence of meta-control in task switching. The first conclusion is that meta-control can be studied with the novel experimental design introduced in this thesis and was found to be reward dependent. The second conclusion is that meta-control acts as a set of parameters that can modulate the mode of control in a sustained way across trials.