The purpose of the present study was to determine if an hour and a half of simulated microgravity using a head-down-tilt model would affect cerebral oxygenation and cognitive performance compared to the supine position. Cerebral oxygenation levels increased between the horizontal and HDT conditions (Figure 3) and that the increase was the largest in the switching condition of the Stroop Task. Furthermore, only the cognitive performance of the executive (Switching) component of the Stroop Task improved during HDT (Figure 3).
Our first hypothesis was that cerebral oxygenation would be highest following an hour and a half of simulated microgravity by using the head-down-tilt (HDT) technique when compared to values obtained in the supine position, which was partially supported by our results. In the current results, cerebral oxygenation increased during measurement consistently across all points it was measured during the post-HDT Stroop Task indicating that this was a global effect. We also found that the difference in cerebral oxygenation post-HDT appeared to be larger in the switching task even though the interaction did not reach significance. Further analysis revealed significant difference pre- to post-HDT in the Switching task but not for the Inhibition task or the Naming task. Similar results for cerebral oxygenation were reported with previous microgravity studies. These studies confirm that simulated microgravity (i.e., parabolic flight, HDT) increases cerebral oxygenation due to the increased blood flow to the head [3, 9] (Kawai et al., 1993 and Schneider et al., 2013). Previous research has shown that increased neural activation can occur in the brain in the presence of increased cerebral blood and oxygen delivery; such as what occurs during microgravity [2, 18] (Bandettini et al., 1997; Lindley et al., 1992). This increased neural activation can create various cognitive advantages as cells and neurons in the brain receive a greater supply of oxygen necessary for cellular metabolism and optimal functioning. These advantages can include faster decision making, decreased reaction time, and increased resistance to neural fatigue  (Bandettini et al., 1997). This increased capacity for cellular functioning might help explain the increased cognitive performance at the end of the HDT in the present study. The increased cerebral oxygenation might have led to increased neural firing frequency resulting in the observed improvement in reaction time. The link between cerebral oxygenation and improvements in cognitive function have been shown before where decision making and reaction time were highest among participants with the greatest levels of cerebral blood delivery  (Ogoh et al., 2014); increasingly difficult executive function tasks required increased levels of cerebral oxygenation  (Williams et al., 2019); and the lowest levels of cognitive performance were correlated with low levels of cerebral blood flow  (Kovarova et al., 2011).
Our second hypothesis was that executive function would improve after an hour and a half simulation of microgravity. This hypothesis was also partially supported by our results. We observed an improvement in the switching condition of the Stroop task. Critically, the switching task targets executive function, while the other two tasks are simpler and require less executive function. Our results are similar to executive function improvements shown with microgravity flights where the microgravity portion led to overall decreases in reaction time, and better problem-solving abilities during the microgravitational phase of their flight  (Wollseiffen et al., 2016). However, other studies have reported contrasting results. Executive functioning via the Iowa Gambling Task was significantly decreased following 60 days of bed rest when compared to the control group  (Lipnicki et al., 2009). Other aspects of executive functioning were also compromised during the bed rest period such as memory, reaction time, and decision making  (Lipnicki et al., 2009). The duration of the intervention is important because of the factors that can influence cognitive performance during sustained microgravity or prolonged bed rest may differ from those of shorter duration microgravity exposures.
It can therefore be concluded that an acute bout of simulated microgravity exposure can momentarily enhance cognition and executive functioning. Based on the findings of the present study and previous research, this improved cognitive performance might be caused by the increased cerebral oxygenation and cerebral blood flow as gravitational force is lost and fluids are redistributed towards the upper portion of the body. However, further research is needed to investigate further mechanisms by which HDT can improve cognition [2, 18, 19] (Lindley et al., 1992; Bandettini et al., 1997, Ogoh et al., 2014).
Limitations of the study
The present study followed many of the accepted methodologies and measurement techniques that currently exist in the literature. However, a few limitations should be noted. First, there were no pre-screening measurements of brain size or total blood volume, which could be responsible for certain variations in cerebral oxygenation. Second, there was no monitoring of cerebral blood flow or cerebral blood flow velocity due to the unavailability of a Doppler Ultrasound device. Thus, there was no way to determine if the variations in cerebral oxygenation were due to cerebral adaptation or varying levels of oxygenated blood reaching the brain. Third, subjects laid flat on their back (i.e., -6° HDT) for the entire treatment period but had to tilt their head sideways to complete the Stroop tasks, which could possibly change the values of cerebral oxygenation. Finally, there was only an hour and a half period between the participants completing the Stroop tasks, which may give speculation to a possible learning effect. However, the Stroop task was heavily reviewed by Jenson & Rohwer (1966), and it was found that there are scarce learning effects with this test due to the lack of correlation between practice and reaction time. Furthermore, increased frequency of the test did not yield significantly better reaction time or accuracy, demonstrating that there is not a substantial learning effect associated with this cognitive task.
Future Areas of Research
The present study demonstrated that short-duration simulated microgravity with a -6°HDT model can significantly increase cerebral oxygenation. This study also found that increased levels of cerebral oxygenation were not statistically associated with increased cognitive performance and executive functioning during simulated microgravity. Future research should examine the effects of longer duration microgravity on cerebral oxygenation and cognition as the present study only used an hour and a half simulation. Next, the present study only monitored cerebral oxygenation at the prefrontal cortex. Future studies should try and measure cerebral oxygenation throughout the entire brain rather than just one area to obtain a broader map of cerebral hemodynamics and oxygenation allocation during testing. Finally, examining cerebral oxygen metabolism at the cellular level to ascertain if the increases in cerebral oxygenation during microgravity simulation are due to increased stress on the brain during cognition or simply due to the increased amount of blood reaching the brain would be very interesting.