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.

The submariners' sleep study: a field investigation of sleep and circadian hormones during a 67-day submarine mission with a strict 6-h-on/6-h-off watch routine.

The submarine working and living environment is an isolated, confined, and extreme (ICE) environment where a continuous on-watch is required to fulfill the tactical objectives. The current study examined whether a physiological and behavioral adjustment to an operational watch standing scheme occurred in terms of hormonal secretion (i.e., melatonin and cortisol) and sleep during a 67-day undersea mission. The crew followed a strict scheme of watch-on blocks at 0:00-06:00 h and at 12:00-18:00 h (group 1, diurnal sleep group) or watch-on blocks at 06:00-12:00 h and 18:00-24:00 h (group 2, nocturnal sleep group). We sampled saliva during the operational blocks over a 24-h period at day 55 of the mission and collected sleep actigraphy data during the entire mission in 10 participants. Sleep showed a biphasic split pattern with significantly unequal distributions of total sleep time (TST) and sleep efficiency (SE) between the two sleeping blocks, i.e., one long and one short sleep bout. Melatonin secretion showed no adjustment at the end of the mission to the watch standing blocks, following an endogenous circadian rhythm independent from the social zeitgebers with indications of a phase shift. Cortisol secretion however matched the biphasic work-sleep shift routine. Human physiology does not fully obey operational needs and there are differences in adjustment between melatonin and cortisol. A watch standing schedule that provides a balance between physiology and operationality still needs to be established. The potential adaptation effects of bright light therapy and melatonin supplementation should be investigated in future research.NEW & NOTEWORTHY The current study has a unique character due to the field context in which the data were collected. Data on long-term submarine missions are scarce and valuable in terms of operational and ecological validity. Moreover, the results are important both from a scientific and operational point of view, showing that human physiology does not fully obey operational needs and that there are differences in adjustment dependent from the type of circadian hormone.

Can Creatine Combat the Mental Fatigue-associated Decrease in Visuomotor Skills?

PURPOSE: The importance of the brain in sports was recently confirmed by the negative effect of mental fatigue (MF) on sport-specific psychomotor skills. Creatine supplementation improves strength but can also improve cognitive functioning. To explore the role of creatine in combating MF, we evaluated whether creatine supplementation counteracts the MF-associated impairment in sport-specific psychomotor skills. METHODS: In 23°C, 14 healthy participants (4 females, 10 males; mean ± SD, age = 24 ± 3 yr, mass = 74 ± 13 kg, height = 179 ± 9 cm) performed a 90-min mentally fatiguing task (counterbalanced, crossover, and double-blinded; i.e., Stroop task) in two different conditions: after a 7-d creatine supplementation (CR; 20 g·d) and after a 7-d calcium lactate supplementation (placebo [PLAC]), separated by a 5-wk washout. In both conditions, a 7-min sport-specific visuomotor task, a dynamic handgrip strength endurance task, and a 3-min Flanker task was performed before and after the mentally fatiguing task. Physiological and perceptual responses were measured throughout the protocol. RESULTS: Handgrip strength endurance was higher in CR compared with PLAC (P = 0.022). MF impaired visuomotor response time (+4.4%; P = 0.022) and Flanker accuracy (-5.0%; P = 0.009) in both conditions. Accuracy on the Stroop task was higher in CR compared with PLAC (+4.9%; P = 0.026). Within the perceptual and physiological parameters, only motivation and vigor (P ≤ 0.027) were lower in CR compared with PLAC. CONCLUSION: Creatine supplementation improved physical (strength endurance) and prolonged cognitive (Stroop accuracy) performance, yet it did not combat MF-induced impairments in short sport-specific psychomotor or cognitive (Flanker) performance. These results warrant further investigation in the potential role of creatine in combating the MF-associated decrements in prolonged (e.g., 90-min soccer game) sport performance and suggest a role of brain phosphocreatine in MF.