Introduction: Maintaining cognitive performance during sleep deprivation is of vital importance in many professions, especially in high-risk professions like the military. It has long been known that sleep deprivation diminishes cognitive performance. To mitigate the negative effects on cognitive performance during crucial military tasks, new interventions are necessary. Non-invasive cervical transcutaneous vagus nerve stimulation (ctVNS) has gained traction as a method to boost alertness and cognitive functioning. Methods: We investigated the effects of a 2 × 2 minute ctVNS stimulation protocol on three cognitive tasks applied during conditions of sleep-deprivation: a psychomotor vigilance task (PVT), a multitasking task (SynWin), and an inhibitory control task (stop-signal task; SST). In addition, participants also performed a close-quarter-battle (CQB) test in virtual reality (VR) to examine if potential effects of ctVNS translate to operational military contexts. A total of 35 military operators from Special Operations Forces (SOF) and SOF support units participated. They were randomly assigned to an active stimulation group or sham group. Before stimulation at 19:00 h, participants performed baseline tests. Participants stayed awake through the night and performed the cognitive tasks every 3 h. The last round of cognitive tasks was followed by the VR test. Results: Though sleep deprivation was successfully induced, as evident from a decline in performance on all three cognitive tasks (effect of session: p < 0.001 SynWin; p < 0.001 PVT; p < 0.001 SST; Linear Mixed Model), no significant effects of ctVNS were found on cognitive task performance, as well as on the military operational VR task. However, the influence of stimulation intensity on SynWin performance showed a trend, indicating that higher stimulation intensities could have a negative impact on cognitive performance. Discussion: A 2 × 2 minute stimulation protocol may not be sufficient to elicit beneficial effects on cognitive-and operational military performance. Moreover, correct stimulation intensity may be critical to induce effects on cognitive performance, as stimulation effects may follow an inverted-u dose-response curve. Stimulation intensities in the current study are higher compared to a similar study that reported beneficial effects of ctVNS, which may explain this null finding. Further research is recommended to optimize stimulation protocols and investigate robustness of effects.

Real-time physiological stress monitoring would be a relevant addition to virtual reality (VR) training for high-risk professions, such as the military. VR is highly suitable for the implementation of such monitoring due to the controlled environment and the already used wearables. However, physiological stress measurements suffer from distortion due to physical activity. Therefore, we tested whether we can use accelerometry to correct non-invasively measured heart rate (HR) for physical activity in 23 soldiers who performed three room-clearing VR scenarios. These scenarios were dynamic, in that soldiers moved around in the VR environment by walking around in the real environment. In contrast to uncorrected HR, and HR corrected by subtracting baseline HR measured when walking, the accelerometry-corrected HR was able to significantly predict the participants’ self-reported stress in the scenarios, p = 0.047, R ² = 0.11. Whereas uncorrected HR significantly predicted self-reported physical demand, p = 0.028, R ² = 0.09, the accelerometry-corrected HR did not. All HR measures significantly predicted self-reported mental effort, which was most strongly the case for uncorrected HR, p < 0.001 R ² = 0.42. These findings, in combination with the methods’ low sensitivity to motion artifacts and non-invasiveness, are very promising for its use to monitor stress in real-time during dynamic VR training scenarios.

The current study explores whether different stressors in a virtual reality (VR) military training scenario cause increases in physiological stress. This would validate the use of VR simulation for stress training, as well as the physiological monitoring of trainees for educational purposes. Military cadets (n = 63) performed a patrol scenario (military convoy) in which they answered questions about their surroundings. Stressors (task difficulty, noise, lighting changes, social evaluations, electric muscle stimulation, and a simulated attack on the convoy) were stepwise added in four phases. Electrocardiogram, blood pressure, electrodermal activity, cortisol, and the cadets’ subjective threat/challenge appraisal were measured. We found that only the first phase caused a significant increase in physiological stress, as measured with heart rate, heart rate variability, and electrodermal activity. Physiological stress appeared to stay high in the second phase as well, but decreased to baseline level in the third and fourth phases, even though these phases were designed to be the most stressful. Cadets classified as threat responders based on physiological data (n = 3) scored significantly higher on subjective threat/challenge appraisal than those classified as challenge responders (n = 21). It seems that in the tested VR training scenario, the novelty of the scenario was the only effective stress stimuli, whereas the other implemented stressors did not cause a measurable physiological response. We conclude that if VR training scenarios are to be used for stress training, these should confront trainees with unpredictable but context-specific demands.