Superior frontal regions reflect the dynamics of task engagement and theta band-related control processes in time-on task effects

Yu, Shijing; Mückschel, Moritz; Beste, Christian · 2022 · Crossref

DOI: 10.1038/s41598-022-04972-y

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Summary

This study investigates the neurophysiological mechanisms underlying time-on-task effects, specifically focusing on how sustained cognitive performance impacts prefrontal cortical activity and task engagement. Motivated by the "opportunity cost model," which posits that mental fatigue arises from the increasing computational costs of maintaining attention in prefrontal networks, the authors examined the interplay between cognitive control and effort. They hypothesized that as time on task increases, the coupling between cognitive control resources and invested effort would decouple, leading to performance declines. To test this, the researchers employed a working-memory modulated response inhibition task combined with electroencephalography (EEG) beamforming and pupillometry. Twenty-seven healthy participants performed Go/Nogo trials with varying rotation angles (30° and 150°) across four sessions. The design allowed for the simultaneous assessment of behavioral performance, theta-band activity (TBA) in specific prefrontal regions, and phasic pupil diameter modulations, which serve as an index of mental effort and task engagement. The analysis focused on comparing the first and last sessions to capture maximal time-on-task effects. Behavioral results confirmed that task performance declined over time, evidenced by reduced hit rates and altered reaction times in later sessions. Neurophysiologically, EEG beamforming revealed a significant decrease in task-evoked theta-band activity in the superior frontal gyrus and supplementary motor area from the first to the last session. Concurrently, phasic pupil diameter modulations, particularly in high-demand conditions, showed a decreasing trend. Crucially, the study found a strong negative correlation between superior frontal TBA and phasic pupil diameter in the initial session, indicating a tight coupling between cognitive control processes and effort investment. This correlation was most pronounced in conditions requiring high inhibitory control and working memory. However, this relationship significantly weakened or vanished by the final session. The findings suggest that time-on-task effects are characterized by a decoupling of cognitive control resources and effort. Initially, high cognitive control demands are matched by high effort investment, reflected in the strong correlation between frontal theta activity and pupil dilation. As the task progresses, this coupling breaks down, implying that while cognitive control mechanisms may still be active, the motivational drive or effort to engage them diminishes. This supports the opportunity cost model, indicating that fatigue in prefrontal functions is not merely a depletion of resources but a strategic withdrawal of effort due to declining expected benefits of continued engagement.

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• neuro workload indices