Near-infrared spectroscopy (NIRS) has become an acceptable, non-invasive method for the quantitative measurement of pre-frontal oxygen status (PFOS; pre-frontal oxy-hemoglobin concentration change), including cognition-related status 1234, and previous studies have demonstrated it to be a reliable and sensitive method in aviation environments 5678910111213. A better understanding of PFOS in terms of cortical signals during actual flight is needed to increase its applicability to flight safety. Moreover, the rapid, accurate, and continuous monitoring of pilot state and/or workload is highly desirable in modern aircraft systems 14. Recently, NIRS has been successfully used to measure the in-flight cerebral oxygen status of F-15 fighter pilots during aerial gunnery training 8 and during air-to-air combat maneuvering 11. Technological advances and a heavy workload have resulted in increasing operational demands on both military and civilian helicopter pilot, so it is critical to understand the pilots' cerebral oxygen status during helicopter flight missions. However, NIRS measurements can be compromised by movements of the head, especially for real world applications, where the movement of the head cannot be restricted, as in studies involving pilots, children, etc. 15. Accordingly, little is known at present about the PFOS (as a cerebral oxygen status) of helicopter pilots during actual flight. In helicopter flight, one study investigated the use of NIRS for critical patients in air medical transport, but this study did not examine the PFOS of the helicopter pilot 9. The aim of this study was to examine cognition-related PFOS in helicopter pilots during actual flight using NIRS with a consideration of motion artifacts.

Four male helicopter pilots in the Japan Air Self-Defense Force (JASDF) volunteered to perform nine sorties with an in-flight examination BK117 and UH-60J single rotor helicopters (Table 1). All the pilots had passed an extensive physical examination within the past 12 months and were healthy. The procedures for this study were approved by the Aeromedical Laboratory Committee in JASDF. All subjects were fully briefed on the scope of the experiment, and written informed consent was obtained from all subjects before the experiment.

The purposes of this study were to: 1) examine the PFOS of helicopter pilots during actual flight using NIRS measurements; and 2) determine whether cognition-related O₂Hb changes could be detected during actual flight. It has previously been shown that NIRS is able to detect changes in blood oxygenation caused by cognitive demands 1234202122. Moreover, NIRS provides a relatively inexpensive and robust monitor for cortical activation, a metric important for determining the cognitive response efficiency for the instantaneous gross mental activity level 23. Hoshi et al. also demonstrated that an increase in O₂Hb showed a positive correlation with task difficulty 24.

However, motion artifacts in NIRS studies are a potentially serious problem for real life applications 15. Motion artifacts such as arise with head movement can cause the blood to move toward (or away from) the area that is being monitored, increasing (or decreasing) the amount of oxygen, hence resulting in an increase (or decrease) in the collected data 15. In this study, accordingly, the effects of head movement on NIRS measurement in healthy subjects were studied under laboratory conditions. The laboratory study showed that the concentration of O₂Hb changed in the range of -1.53 ~ +0.53 μmol•L^-1 from baseline during upward and right/left head movements (Table 2). Thus, there was little change in the O₂Hb concentration, so it is considered that these head movements (upward and right/left) employed in this study will have no affect on PFOS measurement during actual helicopter flight as cognition-related changes. In the laboratory study, however, the concentration of O₂Hb was increased to +3.11 μmol•L^-1 during downward head movement (Table 2). Thus, it was hypothesized that the concentration of O₂Hb with downward head movements would potentially significantly affect the flight study. Therefore head and body movements of the pilots were monitored with a video recorder during the NIRS measurements, except for the case of the artifact caused by the downward head movement of the subjects. Additionally, a researcher rode in the back seat to observe the pilot reactions, and to monitor the downward head movements. The data showed that the period of the downward head movement in helicopter pilots was within a 1~3 second range per head movement (Figure 2, Figure 4, Figure 5, and Figure 6), and then the head position immediately returned to the initial (horizontal) position. As noted above, it was hypothesized that the downward head movement would lead to a motion artifact in the NIRS studies during actual helicopter flight. Although the results of the downward head movement in the laboratory study lead to the conclusion that there could be an abrupt O₂Hb increase, this was not observed during the actual flight study (Figure 2, Figure 4, Figure 5, and Figure 6). There is a possibility that the increase of O₂Hb concentration was a result of cognitive demand (related to task difficulty) during the flight tasks which masked the increase in the O₂Hb levels as an effect of head down movement. The total period of the downward head movement during the flight was observed occurred within 10 seconds to about 10 minutes of flight, which is a relatively long period (Figure 4). Due to the fact that the ratio of the period (downward head movement/~10 minutes flight) is approximately one-sixtieth, it was thought that this movement would be negligible as a motion artifact during the relatively long period of the helicopter flight. However, consideration of the PFOS data from downward head movement may still be important during the relatively short period (i.e., one or two minutes) of the in-flight PFOS study.

On the other hand, the possibility must also be considered that the pilot helmet can change the pressure on the NIRS probes. However, there is no difference in the NIRS measurements between the BK117 pilots without helmets and UH-60J with helmets (Figure 3). In this study, whole body movement, which is mainly comprised of arm and leg movements, resulted in unchanged O₂Hb levels and did not affect the monitoring of the PFOS in helicopter pilots during level flight (Figure 3). Recently, Jayakar et al. reported that mouth movement increased regional cerebral oxygenation values in normal adult subjects while in the supine position 25. However, as shown in Figure 3 (as the pilot talked to the researcher during level flight), increases of O₂Hb concentrations due to mouth movement with verbalization were not detected. This may be due to differences in the measurement conditions, such as a difference in the subjects' position and/or equipment sensitivity.

Gy and Gz were monitored simultaneously during the flight. Gz values during the flight study were within the range of 0.8 to 1.4, and there was little change in Gy. The fact is that we did not observe O₂Hb changes rerated to the Gy and Gz. We conclude that these factors thus did not affect the O₂Hb changes.

Previous research has suggested that the HHb signal is a reflection of O₂ extraction 26. In general, total hemoglobin concentration, which is a measure for blood volume, is defined as the sum of O₂Hb and HHb concentration. In this study, there was comparatively little change in HHb during the measurements of laboratory and flight missions (Figure 1~6, Table 2). These results for HHb might be caused by the fact that the changes in HHb are often much smaller than in O₂Hb. Then, HHb changes were apparently not affected by a heightened cognitive demand of helicopter flights.

In performing the flight measurements, we were not able to successfully measure the absolute tissue hemoglobin saturation (tissue oxygen index) 27. Previously, fMRI-NIRS or PET NIRS simultaneous measurement studies have reported that O₂Hb is strongly correlated with the fMRI signal and rCBF 282930. A number of NIRS measurements of cerebral O₂Hb have demonstration an increased (or decreased) pattern induced by various cognitive tasks 31323334.

Taken together then, the measurement of O₂Hb changes that we observed in NIRS comprises a sensitive method for investigating changes in PFOS in helicopter pilots during flight missions. However, it should be mentioned that the effect of the increased O₂Hb concentration measured in this study is a compensatory mechanism of brain tissue due to the initial deoxygenation caused by the task 35.

In this study, cognitive demands (tasks) during mission flights seem to have had a markedly stronger influence on increases in frontal O₂Hb concentrations than other flight missions, such as those in level flight. Thus, one of the heightened risks of aircraft accidents is probably the reasons for the increase in the cognitive demand (task) component during takeoff, approach-and-landing and strenuous missions. In this study, the increase in concentration of O₂Hb from the basal level during taxi-out/takeoff strongly suggests the O₂Hb concentration is associated with cognitive demand (maneuvers) in helicopter pilots, and this O₂Hb increase in pilots was detectable via NIRS with a monitoring of downward head down movement to eliminate artifacts (Figure 4). In general, it is very difficult for a helicopter pilot to carry out an approach to the top of a mountain. As shown in Figure 5, the concentration of O₂Hb during strenuous missions such as an approach to the top of a mountain also increased from the basal level (see Figure 5, aircraft position from B to D, and during E).

As described above, our in-flight measurements indicate that O₂Hb concentration increases as a function of heightened flying demands. Level flight decreases cognitive demands and, therefore, the concentration of O₂Hb is maintained at a lower level (Figure 3). A previous study reported the right frontal cortex dominates sympathetic activity during stress-inducing mental tasks, including emotional stimuli. However, the findings obtained by various different investigations are contradictory. In several neuroimaging studies, no clear lateralization or functional role of the frontal cortex in automatic nervous system regulation during stress-inducing mental tasks was observed 36. As shown in Figure 2, a similar pattern was detected in PFOS changes between right and left frontal regions of the subjects. The results support the hypothesis that there is no clear lateralization or functional role of the frontal cortex in PFOS during helicopter flight missions.

The present study is one of the first to examine the relationship between actual flight and PFOS in helicopter pilots. The observed increase of O₂Hb appear primarily to be a result of cognitive demand during the flight tasks. The results clearly demonstrate that NIRS provides a sensitive method for the monitoring of cognitive demand in helicopter pilots. Moreover, NIRS measurement with the use of a video-recorder as an artifact eliminator, such as a head down movement, can be used as a monitor of the PFOS during actual helicopter flight.

The authors declare that they have no competing interests.

AK participated in the sequence alignment and drafts the manuscript. AK participated in the design of the study. YM conceived of the study and in its design and coordination and helped to draft the manuscript. All authors read and approved the final manuscript.