Introduction Both Mental Fatigue (MF) and hypoxia impair multiple aspects of cognitive functioning. The decline in cognitive functioning in hypoxic conditions is associated with alterations in brain oxygenation and hemodynamic responses. These hemodynamic responses are preferably measured at the prefrontal cortex, an area of the brain that is known for its executive function and role in decision making, planning, attention and (short-term) memory. This study will investigate the role of prefrontal cortex oxygenation during the development of mental fatigue and during cognitive performances by altering the ambient oxygen availability through normobaric hypoxia (3800m; 12,9% O2) and normoxia.

Methods Subjects will perform four trials in a sound-insulated climate chamber (20°C and 40% RH). Upon entry in the climatic chamber participants will adapt to the environment for 30 minutes. Next, they will perform a modified cognitive test battery "cognition", a fine motor task "Motor Performance Series" and a visuomotor-fitlight task before and after a 60-minute individualized Stroop task or control task (randomized. blinded, placebo controlled, counter-balanced, cross-over design). Nearinfrared spectroscopy (NIRS) will be used to assess hemodynamic changes (oxygenated hemoglobin (O2Hb), deoxygenated-hemoglobin (HHb) and total hemoglobin (tHb)) at the PFC.

Hypotheses 1) MF will lead to earlier changes in the prefrontal NIRS-parameters (O2Hb, HHb, tHb) with lower oxygen availability. 2) The effects of MF on cognitive performance manifest itself to a greater extent with lower oxygen availability.3) Visuomotor performance declines to a greater extent due to MF with lower oxygen availability.

Inclusion Criteria:

• Healthy (No neurological or cardiovascular disorders)
• Male or female
• No medication
• Non-smoker
• Between 18 and 35 years old
• Recreational athlete population; performance level 2 or 3 for men according to De Pauw et al. (2013)[29] and performance level 2 or 3 for woman according to Decroix et al. (2015)
• Non-acclimatized to altitude (at least 2 months)

Exclusion Criteria:

• Injuries
• Acclimated to altitude
• Use of medication
• Use of caffeine and heavy efforts 24 hours prior each trial
• Not eating a standardized meal, the morning of each trial

We will inform the participants in a way that explains the study as an investigation about the influence of different cognitive tasks on a subsequent physical task at various altitudes looking at other physiological outcome measures.