Investigating the Influence of prolonged Stroop task on mental fatigue and the consequences on corticospinal and corticocortical excitability: A pilot study

Changes in the corticospinal (CSE) and cortico-cortical (CCE) excitability of the primary motor cortex (M1) may underlie the effect of mental fatigue on physical performance. To date, research on this subject has predominantly focused on the examination of CSE, with limited exploration of effects of mental fatigue on CCE. This study aims to investigate the influence of mental fatigue induced through prolonged cognitive activity on both CSE and CCE. Fifteen healthy adults (aged 29.13{+/-}7.15 years) participated in assessments of CSE (Motor evoked potential - MEP amplitude) and CCE (Intracortical facilitation - ICF, short-interval intracortical inhibition - SICI, and long-interval intracortical inhibition - LICI) before and after a 60-minute Stroop task (experimental condition) or watching a documentary (control condition). Subjective mental fatigue was measured using the mental fatigue visual analogue scale (M-VAS), and workload associated with the tasks was assessed using the National Aeronautics and Space Administration (NASA) task load index. Objective mental fatigue was defined by the time-related decline in Stroop task performance. The study results revealed no significant differences in M-VAS, CSE and CCE between the two conditions. Stroop task performance did not exhibit significant changes over time. However, participants perceived the Stroop task to be more mentally demanding and effortful than watching the documentary (p<0.05). Further analysis of Stroop task performance at individual participants level identified two sub-groups of participants: one exhibiting deteriorating performance with time (fatigued subgroup) and the other showing improved performance (non-fatigued subgroup). Descriptively, cortical inhibition increased (reduced SICI and ICF values) from pre to post Stroop task in the fatigued subgroup, while the non-fatigued group displayed an opposite pattern. The findings suggest that mental fatigue may lead to increased cortical inhibition, highlighting the need for further investigation with a larger sample size.