A drop in cognitive performance, whodunit? Subjective mental fatigue, brain deactivation or increased parasympathetic activity? It's complicated!

Introduction: The discussion on the mechanism(s) underlying mental fatigue is still ongoing. We want to reconfirm a performance-impairing effect of executing a lengthy cognitive task on the subsequent task, and determine how this effect is subtended by neurophysiological variations and subjective experience. Methods: Twenty participants (12 females; age: 23 ± 1 y) performed an experimental (EXP) and a control trial (CON) in a randomized counter-balanced order. In both trials a 90-min cognitive task had to be performed (EXP, Stroop task; CON, documentary), that was preceded and followed up by a 10-min flanker task that was completed in the MRI scanner. Throughout the protocol, subjective self-evaluation, peripheral autonomic activation and metabolic measures, cognitive performance and functional brain imagery were recorded. Due to equipment issues, only 11 participants could be included in the analysis of the peripheral autonomic activation. Results: Flanker performance dropped both in EXP and CON (p =.010). Heart rate variability increased in time, both in EXP and CON (p ≤.047). A time-on-task related drop in Stroop performance (p =.007) and higher subjective mental fatigue was observed in EXP compared to CON (p <.001). Moreover, the BOLD signal of response inhibition-associated brain activity in corpus callosum, somatosensory association cortex and anterior cingulate cortex was reduced during the post-flanker task in EXP compared to CON (p <.001). Discussion Our results indicate two different processes: 1) A time-on-task effect as a peripheral physiological deactivation that coincided with the observed post-flanker performance drop both in EXP and CON; and 2) An increase in the level of subjective mental fatigue with prolonged performance on a 90-min Stroop task that is associated with a decrease in response inhibition-associated brain activity in both grey and white matter, specifically in the EXP-condition. Conclusion: Our results highlight the multifactoriality of carryover effects, in the present study increased parasympathetic activity was linked with the drop in performance.