Description: The way mental load and acute stress affect brain activity during complex tasks, like piloting an aircraft, is an important concern in applied neuroscience. The present fMRI study aimed at highlighting patterns of brain activations and autonomic activity (heart rate, pupil diameter) involved in coping mechanisms and/or task engagement when confronted with high cognitive load and acute stress. Twenty young healthy adults performed a complex cognitive task (mental calculation n-back task, 2 levels of difficulty) in either safe or aversive (threat of auditory stressors) conditions. Our results showed that mental effort induced recruitment of the fronto-parietal executive control network (ECN), along with disengagement of the default mode network (DMN), including the ventromedial prefrontal cortex and posterior cingulate regions. Mental effort also elicited an increase in heart rate and pupil diameter. Task performance did not decrease under stress, most likely due to efficient coping mechanisms. When stress was induced, large decreases of activity in superior temporal, medial prefrontal, insular and amygdala regions were observed, along with an increase of lateral prefrontal and parietal activations. This may reflect coping mechanisms aimed to reduce the impact of the auditory stressors, inhibiting the salience network (SN) for emotional regulation and disengaging the DMN, while enhancing ECN activity for improved attentional and cognitive processes towards the cognitive task. Regarding practical neuroergonomic applications, measuring specific prefrontal activation patterns reflecting the balance between ECN, DMN and SN could be used for objective in situ mental state assessment in safety-critical domains. In this sense, a high ratio of lateral vs. medial prefrontal activity may represent a relevant marker of increased but efficient mental effort while the opposite may indicate a disengagement from the task due to mental overload and/or acute stress.
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