The paradoxical relationship of difficulty and lateral frontal cortex activity

Title The paradoxical relationship of difficulty and lateral frontal cortex activity. Competing financial interests The authors declare no competing financial interests. Running Title Rostrocaudal reaction time effects Abstract Task difficulty is widely cited in current theory regarding cognitive control and fronto-­‐parietal function. Ongoing debate surrounds the extent to which global difficulty across multiple cognitive demands is the main driver of lateral frontal activity. Here, we examine a commonly cited behavioral marker of difficulty in these accounts: time-­‐on-­‐task (ToT), as assessed by response time. Specifically, we investigate the task-­‐dependent scaling of frontal BOLD responses with ToT during hierarchical cognitive control. We observe a paradoxical relationship, whereby rostral regions show greater scaling with ToT on a first-­‐order task, despite showing greater recruitment on a second-­‐order task; caudal regions show the converse relationships. Together, these results demonstrate that ToT does not reflect a single dimension of difficulty that uniformly drives lateral frontal activity. Rather, this discrepancy in the mean and scaling of BOLD requires that multiple, distinct processes are instantiated across these fronto-­‐parietal regions in the service of cognitive control function.