Abstract
Mental effort is a common phenomenological construct deeply linked to volition and self-control. While it is often assumed that the amount of exertion invested in a task can be voluntarily regulated, the neural bases of such faculty and its behavioural effects are yet insufficiently understood. In this study, we investigated how the instructions to execute a demanding cognitive task either “with maximum exertion” or “as relaxed as possible” affected performance and brain activity. The maximum exertion condition, compared to relaxed execution, was associated with speeded motor responses without an accuracy trade-off, and an amplification of both task-related activations in dorsal frontoparietal and cerebellar regions, and task-related deactivations in default mode network (DMN) areas. Furthermore, the visual cue to engage maximum effort triggered an anticipatory widespread increase of activity in attentional, sensory and executive regions, with its peak in the brain stem reticular activating system. Across individuals, this surge of activity in the brain stem, but also in medial wall cortical regions projecting to the adrenal medulla, positively correlated with increases in heart rate, suggesting that the intention to willfully modulate invested effort involves mechanisms related to catecholaminergic transmission and a suppression of DMN activity in favor of externally-directed attentional processes.
Highlights
Mental effort is a common phenomenological construct deeply linked to volition and self-control
We investigated the presence of across-subjects correlations between EXR-RLX differences in response times (RT), Heart rate (HR) values, and TLX-NASA ratings
The voluntary engagement of maximum exertion resulted in faster response times, compared to performing the task in a relaxed fashion, with the effect being significantly stronger for the congruent trials
Summary
Mental effort is a common phenomenological construct deeply linked to volition and self-control. The relationship among the willful application of mental effort, its subjective perception, and its effects on cognitive performance, as well as the neural circuitry underlying effort processing, remains a topic insufficiently understood to this date (but see Shenav et al.[16], for an excellent overview of recent progresses) In his seminal work, “Attention and effort”, Daniel Kahneman proposed the equivalence of the problem of mental effort to that of the allocation of limited attentional resources to a demanding task[17]. Kahneman considered the investment of effort in a task as a largely automatic process, dependent on task difficulty, rather than a mental factor that could be applied in a graded fashion by an act of will While this approach had the great benefit of operationalizing effort, it left the aspects of voluntary control and subjective perception of effort, along with their neural substrates, virtually unexamined. The anterior and mid-cingulate cortex (ACC, MCC) in particular, whose activity tends generally to increase with the difficulty of a task[20,21], has been proposed as a key structure in cognitive control for the detection of performance errors[22,23], for signalling response conflict[24,25,26], and for encoding effort[27,28,29], its precise role is yet to be unambiguously determined[30,31], perhaps because of its functional and structural heterogeneity[32]
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