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Stroop performance of partially sleep-deprived healthy volunteers at simulated hypobaric hypoxia condition
, Deepak Kumar Khukhar1, Seema Gambhir1, Harshavardhan Gandikota1, AVK Raju1, Ranjan Sarkar1, Sachin Bharadwaj2
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Received: ,
Accepted: ,
How to cite this article: Mohapatra SS, Khukhar DK, Gambhir S, Gandikota H, Raju A, Sarkar R, et al. Stroop performance of partially sleep-deprived healthy volunteers at simulated hypobaric hypoxia condition. Indian J Aerosp Med. doi: 10.25259/IJASM_26_2025
Abstract
Objectives:
The amount of average nightly sleep that the military aircrew gets is 6 h, which is 2 h less than the optimal sleep of 8 h. With the background that the executive type of cognitive functions, which are critical to flight operations, deteriorate under hypoxic conditions, it was a purposeful initiative to undertake the study to determine if the cognitive dysfunction related to executive functions is further deteriorated by 2 h of partial sleep deprivation (PSD) or not.
Material and Methods:
The experiment was carried out on 25 healthy participants on two occasions, i.e., on the day after an optimal sleep and on the day after 2 h of sleep deprivation at three different pressure altitudes, i.e., ground level, 10,000 ft and 18,000 ft in a hypoxia simulator explosive decompression chamber. The Stroop test was selected from the psychological test battery called Psychomotor Evaluation Designed for Aviators (pSuMEDhA) developed at the Institute of Aerospace Medicine. SpO2 was measured using a finger pulse oximeter. Two-way ANOVA was conducted to compare the means.
Results:
“Altitude” effect in the form of statistically significant fall of mean SpO2 from average ground level values of (Day-1 and Day-2) 97.8% and 98% to 92.1% and 92.6% at 10,000 ft, and 79.1% and 78.9% at 18,000 ft was observed. Similarly, a statistically significant increase of mean Stroop time (ST) from an average ground-level value of 11.04 and 11.92 to 19.08 and 26.36 at 10,000 ft and 43.28 and 55.68 at 18,000 ft was observed on both days. However, there was neither the “Day” effect nor the “Interaction (Day Altitude)” effect.
Conclusion:
ST, which is the indicator of cognitive difficulty, was higher at 18,000 ft in comparison to the ST recorded at ground and at 10,000 ft observed on the day after optimal sleep and also on the day after 2 h of PSD. When ST at different levels of altitude was compared, the values recorded on Day-2 were not statistically different from the values recorded on Day 1, implying similar cognitive difficulty. The accentuation effect of “Two hours PSD” on cognitive performance under hypoxia condition could not be established through Stroop testing.
Keywords
Cognitive function
Hypobaric hypoxia
Sleep deprivation
Stroop test
INTRODUCTION
Hypoxia, especially the hypobaric hypoxia, is generally recognized to be the most serious single physiological hazard during flight at altitude.[1] The impairment of psychological functions, viz., mental, psychomotor and cognitive performances, produced by exposure to reduced oxygen at altitude is of great practical and operational significance in aviation.
Recent findings by Taylor et al. highlight the detrimental effects of hypoxia on cognitive performance.[2] Complementarily, research indicates that acute hypobaric hypoxia can lead to significant declines in various psychophysiological metrics.[3] Generally, the execution of well-learned and practiced tasks remains relatively preserved at altitudes up to approximately 10,000 feet. However, when alveolar oxygen levels decrease to below 38–40 mmHg, corresponding to altitudes exceeding 16,000–18,000 feet, significant impairments in psychomotor and cognitive functions are observed. Evidence demonstrates that hypoxic conditions adversely impact several cognitive domains, including simple reaction times, pursuit tasks, choice reaction times, complex eye-hand coordination, muscular coordination, conceptual reasoning, as well as both short-term and long-term memory.[1] Moreover, studies have documented pronounced negative effects on central executive functions under hypoxia, while its impact appears to be less pronounced on nonexecutive functions, perception, attention, and short-term memory tasks.[4]
Investigators had used various tests to assess the cognitive function under hypoxia condition. While the tests such as Digit Span Forward and Backward tests and Trailmaking A and B tests were used to assess the short-term memory and working memory function under hypoxia condition,[5-8] the Stroop test[9,10] was used to assess the executive function under similar conditions.[7]
It is also well documented that adequate sleep is necessary for optimal cognitive and psychomotor function. Partial sleep deprivation (PSD) or total sleep deprivation can lead to impairment in alertness, attention, vigilance and performance related to executive functions[11] such as flexible thinking,[12] inhibitory behaviour,[13] decision making,[14,15] judgment,[16] risk-taking tendency etc.[17] Functional neuroimaging studies had confirmed the disruptions of real time glucose metabolism in the prefrontal and anterior cingulate cortices are primarily responsible for deterioration of executive function in the sleep deprived personnel.[18,19] Earlier studies have confirmed that tests such as the Controlled Oral Word Association test, the Stroop test, Trail-making tests could be used practically to assess the PSD-induced cognitive dysfunction.[20]
Executive function being the common area of affection to both hypoxia and sleep deprivation, it was for the interest of the research to investigate if PSD has any additive influence on the cognitive deterioration due to hypoxia, thus compounding the problems further causing greater concern to aerospace safety. Present available literature does not throw much light on combined effect of hypoxia and sleep deprivation on cognitive functions. This has relevance in the armed forces as the aircrew are susceptible to both hypoxia and short spells of sleep deprivation. A recent study confirmed that the amount of night sleep of a military aircrew is about 6 h, which is 2 h less than the optimal sleep of 8 h.[21] With the background that the executive type of cognitive functions, which are critical to flight operations gets deteriorated under hypoxic conditions, it was a purposeful initiative to undertake the study to determine if the cognitive dysfunction related to executive functions are getting deteriorated further by 2 h of PSD or not. The Stroop performance was assessed during the simulated hypoxia condition among the healthy individuals with and without 2 h PSD.
MATERIAL AND METHODS
Study design
The study was a cross-sectional observational study having a within-subject (repeated measure) study design with randomly selected healthy adult volunteers, who were selected using a convenient sampling method. The study was carried out in the Department of High Altitude Physiology at the Institute of Aerospace Medicine (IAM) in Bangalore (India). The ethical clearance was obtained by the Institute Ethics Committee. Informed written consent was obtained from each participant.
Participants
25 healthy volunteers of either sex in the age group of 20– 40 years were randomly selected using a convenient sampling method. The estimated sample size calculated by G power software (version 3.1.9.4.) by considering a “Repeated-measures-within-factor ANOVA test” having 2 groups and 3 measurements with an “effect size” of 0.35 and “power” of 0.95 was 24. The Exclusion criteria considered were (a) History of any co-morbidity, including grossly overweight/obesity, (b) Any sleep disorders, (c) Undertaking any sedative or sleep-inducing medication, (d) Respiratory illness, and (e) History of claustrophobia since the study included the exposure of the participants to hypobaric hypoxia in the explosive decompression chamber (EDC).
Materials
The hypobaric chamber utilized in this study was custom-designed for the Indian Air Force (IAF) by KASCO Industries Ltd, located in Pune, India [Figure 1]. This chamber comprises two distinct compartments: the Main Chamber (MC) and the Air Lock Chamber (ALC). The MC is equipped with ten seating positions, while the ALC accommodates four seats, each with independent oxygen delivery systems. The MC is capable of simulating altitudes up to 50,000 feet, whereas the ALC can reach altitudes of 100,000 feet. Furthermore, the chamber is equipped with facilities for both manual and automated operation, controlled via a centralized control unit [Figure 2].
The Neuro Cognitive Battery “pSuMEDhA” was utilized to conduct the Stroop Test. Developed by the Indian Air Force (IAF) as a computer-assisted cognitive test battery, pSuMEDhA assesses critical cognitive and psychomotor skills essential for aircrew performance, such as attention, memory, coordination, vigilance, and threat perception.[22] This indigenous tool provides a standardized evaluation framework, thereby enhancing the assessment of competencies crucial for effective flying operations. The adoption of such a tool signifies a noteworthy advancement in the fields of aviation psychology and aircrew training methodologies.
The Stroop Test was administered in two distinct phases. In the first phase, participants were tasked with matching the word displayed in a box with the corresponding same-colored square presented below. The ink color matched the word. Matching was executed by clicking on the appropriately colored square. During the second phase, participants were required to match the ink color of the word to the same-colored square, with the ink color differing from the word itself. The colors utilized for both the words and squares included red, green, yellow, and blue. Each word appeared six times, resulting in a total of 24 words (6 × 4) presented in a random order over a span of 3 minutes for each phase. Each word was displayed on the screen for 1 second, with the subsequent word appearing immediately thereafter. Participants were instructed to read each word as rapidly as possible and provide a response. The Stroop reaction time was calculated as the mean of all correct response times measured in milliseconds. Stroop Time (ST) was determined by subtracting the Stroop reaction time of phase 2 from that of phase 1, with positive ST values indicating an increased level of difficulty in phase 2. The cognitive domain assessed by the Stroop Test primarily pertains to “Executive Function,” specifically evaluating “Response Inhibition” and “Selective Attention.” Notably, the dorsolateral prefrontal cortex and anterior cingulate cortex—regions responsible for executive functions—are commonly affected by both environmental hypoxia and sleep deprivation.
Protocol
All participants received comprehensive information regarding the study and provided consent to participate. A three-day sleep history was collected to assess the average sleep duration of each subject. Testing on Day 1 was performed after confirming that participants had achieved optimal sleep. On Day 2, testing occurred following a 2-hour reduction in their typical duration of restful night sleep. All experimental procedures were conducted at 0900 hours, ensuring that none of the participants had consumed alcohol within the previous 12 hours. Additionally, they refrained from ingesting coffee, tea, smoking, or engaging in strenuous physical exercise within 2 hours prior to exposure to hypoxia in the Environmental Decompression Chamber (EDC).
A standardized Ear Clearance Run (ECR) protocol was employed, which included a maximum altitude of 10,000 feet, alongside graded ascent and descent rates, with a prescribed maximum rate of ascent and descent of 3000 feet per minute. The ECR was deemed successful when participants exhibited a normal tympanic membrane during the Ear, Nose, and Throat (ENT) examination post-protocol, and reported no subjective symptoms indicative of Otitic or Sinus Barotrauma throughout the ECR procedure.
Following a successful ECR, participants undertook the Stroop Test at three different altitudes: first at ground level, second at 10,000 feet, and third at 18,000 feet within the EDC, maintaining the ascent rate of 3000 feet per minute while inhaling normal chamber air to simulate hypobaric hypoxia. This identical protocol was replicated for each participant on Day 2, during which they experienced 2 hours of sleep deprivation the preceding night.
Hypothesis testing
The main hypothesis of the study was that there would be an accentuation effect of 2 h PSD on the cognitive performance deterioration under hypoxic conditions. The null hypothesis, which was considered for the statistical analysis, was that there are no such effects.
Statistical analysis
Statistical analyses were conducted by using MS Excel®, MS Word® and IBM Statistical Package for Social Sciences (version 20) statistical programming software. After testing the data for “Normality” by Shapiro–Wilk test and “Homogeneity” by Levene’s test, two-way repeated measures analysis of variance (ANOVA) was conducted to find out the changes in the cognitive parameters in the ST at different altitudes and on different days. The post hoc analysis was carried out by the Ryan–Einot–Gabriel–Welsch Range (REGWR) test to make the pair-wise comparisons to determine the pair having significant differences in their means. The level of statistical significance was kept at P < 0.05.
RESULTS
Of the total 25 participants, 19 were males and 06 were females. The participants were in the age group of 27–43 years. The mean and median age of the participants is 39 years. The other demographic profile of the participants is presented in Table 1.
| Demographic details | Category | Number | Percentage |
|---|---|---|---|
| Age | 20–30 years | 10 | 40 |
| 30–40 years | 11 | 44 | |
| >40 years | 4 | 16 | |
| Sex | Male | 19 | 76 |
| Female | 6 | 24 | |
| Marital status | Single | 21 | 84 |
| Married | 4 | 16 | |
| Smoking | Yes | 4 | 16 |
| No | 21 | 84 | |
| Alcohol | Yes | 15 | 60 |
| No | 10 | 40 |
The Stroop test was conducted after confirming the onset of hypoxia by recording the fall of SpO2 below 90%. The mean and standard deviation for both these variables are displayed in Table 2.
| Day | Day 1 | Day 2 | ||||
|---|---|---|---|---|---|---|
| Altitude | Ground | 10K ft | 18K ft | Ground | 10K ft | 18K ft |
| SPO2 (%) | ||||||
| Mean | 97.8 | 92.1 | 79 | 98 | 92.6 | 78.9 |
| SD | 0.8 | 1.7 | 4.6 | 0.8 | 1.6 | 4.9 |
| ST (ms) | ||||||
| Mean | 11.04 | 19.08 | 43.28 | 11.92 | 26.36 | 55.68 |
| SD | 38.07 | 36.09 | 42.99 | 29.86 | 39.86 | 58.74 |
ST: Stroop time, PSD: Partial sleep durations, SD: Standard deviation
Changes in SpO2
With an increase in altitude, a gradual fall of mean SpO2 was observed on both days [Figure 3]. In order to confirm this phenomenon and to determine if the level on 2nd day was different from that on Day 1, Two-Way ANOVA was carried out for the dependent variable SpO2. The results are tabulated in Table 3. The “Main Effect (ME)” of altitude was significant (at df = 2, F = 540.2, p < 0.001 and Effect Size = 0.88). The post hoc analysis carried out by REGWR test had confirmed the significant differences in the pair Ground_10K, Ground_18K and 10K_18K with p < 0.001 for all three [Table 4]. ME of Day (day after optimal sleep vs. day after 2 h PSD) were not significant (at df = 1, F = 0.17, p = 0.67 and Effect Size = 0.001). The “Interaction Effect” of “Altitude_Day” on SpO2 was also not significant (at df = 2, F = 0.11, p = 0.88 and Effect Size = 0.002).
| Source | Dependent variable | Type III sum of squares | Degrees of freedom | Mean square | F | p-value | Partial eta squared | Observed powerc |
|---|---|---|---|---|---|---|---|---|
| Corrected model | ST | 40701.413a | 5 | 8140.283 | 4.637 | 0.001 | 0.139 | 0.972 |
| SpO2 | 9459.740b | 5 | 1891.948 | 216.223 | p < 0.001 | 0.882 | 1 | |
| Intercept | ST | 116705.7 | 1 | 116705.7 | 66.478 | p < 0.001 | 0.316 | 1 |
| SpO2 | 1209067 | 1 | 1209067 | 138179.1 | p < 0.001 | 0.999 | 1 | |
| Altitude | ST | 38107.25 | 2 | 19053.63 | 10.853 | p < 0.001 | 0.131 | 0.99 |
| SpO2 | 9456.16 | 2 | 4728.08 | 540.352 | p < 0.001 | 0.882 | 1 | |
| Day | ST | 1761.307 | 1 | 1761.307 | 1.003 | 0.318 | 0.007 | 0.169 |
| SpO2 | 1.5 | 1 | 1.5 | 0.171 | 0.679 | 0.001 | 0.07 | |
| Altitude*Day | ST | 832.853 | 2 | 416.427 | 0.237 | 0.789 | 0.003 | 0.087 |
| SpO2 | 2.08 | 2 | 1.04 | 0.119 | 0.888 | 0.002 | 0.068 | |
| Error | ST | 252798.9 | 144 | 1755.548 | ||||
| SpO2 | 1260 | 144 | 8.75 | |||||
| Total | ST | 410206 | 150 | |||||
| SpO2 | 1219787 | 150 | ||||||
| Corrected total | ST | 293500.3 | 149 | |||||
| SpO2 | 10719.74 | 149 |
aR Squared=0.139 (Adjusted R Squared=0.109), bR Squared=0.882 (Adjusted R Squared=0.878), cComputed using alpha=0.05. ST: Stroop time, PSD: Partial sleep durations, ANOVA: Analysis of variance
| Dependent Variable | (I) Altitude | (J) Altitude | Mean difference (I-J) | Standard error | p-valueb | 95% Confidence interval for differenceb | |
|---|---|---|---|---|---|---|---|
| Lower bound | Upper bound | ||||||
| ST | Ground | 10K ft | −11.24 | 8.38 | 0.546 | −31.538 | 9.058 |
| 18K ft | −38.000* | 8.38 | 0.000 | −58.298 | −17.702 | ||
| 10K ft | Ground | 11.24 | 8.38 | 0.546 | −9.058 | 31.538 | |
| 18K ft | −26.760* | 8.38 | 0.005 | −47.058 | −6.462 | ||
| 18K ft | Ground | 38.000* | 8.38 | p < 0.001 | 17.702 | 58.298 | |
| 10K ft | 26.760* | 8.38 | 0.005 | 6.462 | 47.058 | ||
| SpO2 | Ground | 10K ft | 5.560* | 0.592 | p < 0.001 | 4.127 | 6.993 |
| 18K ft | 18.920* | 0.592 | p < 0.001 | 17.487 | 20.353 | ||
| 10K ft | Ground | −5.560* | 0.592 | p < 0.001 | −6.993 | −4.127 | |
| 18K ft | 13.360* | 0.592 | p < 0.001 | 11.927 | 14.793 | ||
| 18K ft | Ground | −18.920* | 0.592 | p < 0.001 | −20.353 | −17.487 | |
| 10K ft | −13.360* | 0.592 | p < 0.001 | −14.793 | −11.927 | ||
Based on estimated marginal means. *The mean difference is significant at the 0.05 level, bAdjustment for multiple comparisons: Bonferroni. ST: Stroop time, PSD: Partial sleep durations, ANOVA: Analysis of variance
Stroop performance
With an increase in simulated altitude, a gradual increase in test value of ST was observed on both days, and this was confirmed by Two-Way ANOVA [Table 3; Figure 4]. The ME of altitude on ST was significant (at df = 2, F = 10.8, p < 0.001 and Effect Size = 0.131). With “REGWR” post hoc analysis, the pairs of altitudes having significant differences in mean ST were Ground_18K and 10K_18K with p < 0.001 and 0.005 respectively [Table 4]. ME of Day on ST was not significant (at df = 1, F = 1.0, p = 0.31 and Effect Size = 0.007). The “Interaction Effect” of “Altitude_Day” on ST was also not significant (at df = 2, F = 0.23, p = 0.78 and Effect Size = 0.002).
Thus, the effect of altitude was evident as the mean ST at 18,000 ft was significantly higher than the mean ST recorded at ground level as well as the level recorded at 10,000 ft. However, the effect of 2 h PSD on the Stoop test was not confirmed as the mean ST recorded at different altitudes on Day 2 was not statistically different from those recorded on Day 1. Similarly, there was no interaction effects of altitude and sleep deprivation on SpO2 or ST. Thus, the null hypothesis (no effect will be seen) could not be rejected for Stroop performance, and therefore, it could be implied that the cognitive challenge due to 2 h PSD was insufficient to accentuate the cognitive deterioration under hypobaric hypoxia.
DISCUSSION
In-flight hypoxia remains a critical threat to crew performance and overall flight safety. Despite advancements in supplemental oxygen use and cabin pressurization, cognition suffers under hypoxic conditions. This phenomenon is supported by literature on hypoxia’s detrimental effects on attention, vigilance, and higher cognitive functions.[11] Sleep deprivation is known to further impair cognitive capacities, making the potential combined effects of hypoxia and sleep deprivation particularly relevant in high-stress military aviation scenarios, where aircrew may experience PSD.
Our study aimed to elucidate whether Partial Sleep Deprivation (PSD) would exacerbate cognitive impairments induced by hypoxia. The results indicated that while hypoxia significantly impaired cognitive performance as measured by the Stroop Test, the expected exacerbation of cognitive decline due to 2-h of PSD did not manifest. This finding suggests that a brief period of sleep deprivation was insufficient to further diminish cognitive performance within the hypoxic environment encountered.
Previous literature underscores the profound effects of altitude on cognitive tasks, with significant degradation in cognitive performance anticipated at greater altitudes due to diminished oxygen availability, which can impair critical brain regions responsible for cognitive processing.[23-26] While our findings are consistent with the established understanding of hypoxia, they present a divergence concerning the interaction with sleep deprivation. Other investigations into the effects of sleep deprivation on Stroop performance have yielded mixed results, with some studies reporting no significant cognitive decline even after extended periods of wakefulness.[27-29] Our research reinforces these observations, demonstrating that a mere deficit of 2-h of sleep does not result in a substantial cognitive decline under hypoxic conditions.
Although we initially hypothesized that 2-h of PSD would exacerbate cognitive outcomes compared to a fully rested state, our study did not support this assumption. The findings suggest that optimal cognitive functioning can be maintained even with brief periods of sleep deprivation during moderate hypoxia. Nevertheless, this conclusion necessitates further rigorous investigation. Future research should involve a larger and more diverse sample size, evaluate a broader spectrum of cognitive functions beyond the Stroop Task, and consider individual differences such as sleep propensity (e.g., morning types versus evening types). These variables may help clarify how differing sleep patterns interact with cognitive performance in hypoxic conditions.
In summary, while hypoxia alone significantly impairs cognitive functions, the short-term effects of sleep deprivation may not exert additional detrimental effects within the parameters tested. Continuous exploration of this interplay is essential to enhance safety protocols in aviation and other critical operational settings.
Limitations of the study
A significant limitation of this study is the lack of empirical baseline data on the average daily sleep duration of participants in the 3 days before the experiment. Instead of collecting actual sleep history, the study relied on two assumptions: First, a pre-defined “optimal sleep requirement” of 8 h for military aircrew based on previous literature, which may not accurately reflect the individual needs of all participants; and second, the assumption that participants achieved this optimal sleep duration on Day 1, with a subsequent reduction of 2 h for Day 2. This methodological approach introduces uncertainty regarding the true sleep patterns, limiting our ability to accurately evaluate the relationship between sleep duration and cognitive performance under hypoxic conditions. Future research should implement systematic methods for capturing actual sleep data to enhance the understanding of how sleep impacts cognitive outcomes in similar scenarios.
CONCLUSION
The study demonstrates that hypobaric hypoxia significantly impairs executive cognitive function, particularly at an altitude of 18,000 feet, as indicated by increased Stroop time. Notably, 2-h of PSD did not further exacerbate these cognitive impairments. While altitude continues to pose a primary physiological threat to flight safety, minor sleep deficits did not have a synergistic detrimental effect on cognitive performance under the conditions examined. These results suggest that aircrew exhibit resilience against minor sleep loss; however, the risks associated with hypoxia remain a critical concern.
Ethical approval:
The research/study was approved by the Institutional Review Board at the Institute of Aerospace Medicine, number 4838/2017, dated 25 May 2016.
Declaration of patient consent:
The authors certify that they have obtained all appropriate patient consent forms. In the form, the patients have given their consent for their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.
Conflicts of interest:
There are no conflicts of interest.
Use of artificial intelligence (AI)-assisted technology for manuscript preparation:
The authors confirm that they have used artificial intelligence (AI)-assisted technology solely for language refinement and to improve the clarity of writing. No AI assistance was employed in the generation of scientific content, data analysis or interpretation.
Financial support and sponsorship: Nil.
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