Mental Work Load Assessment during different Simulated Instrument Meteorological Conditions, in Clouds and During Dark Night

Introduction

Spatial Disorientation (SD) is one of the major causes of human error accidents in aviation, reportedly resulting in “at least 25-33% of all aircraft mishaps” [1, 2]. It is defined as “a variety of incidents occurring in flight in which pilot fails to sense correctly the position, motion or attitude of the aircraft or of himself within the fixed coordinate system provided by the surface of the Earth and the gravitational vertical. In addition, errors in perception by pilots of their position, motion or attitude with respect to their aircraft or of their own aircraft relative to other aircraft” [1].

SD mostly occurs when ambient visual cues are denied in flight. Conflicting inputs from nonvisual sensory modalities deceive the pilot to believe what the body feels while he ignores what the instruments read. Flying, during bad weather or at night, adds to the mental workload. Such flights also increase pilots’ susceptibility to SD. An assessment of in-flight workload may help determine the available cognitive resources in task performance [3]. In one such early work, Roman studied the cardio-respiratory response of three aviators to the in-flight environment of F-100 combat aircraft [4]. The subjects were found to have higher blood pressure (BP) and normal or near normal pulse and respiration rate (RR) during cross-country flights including sorties during marginal Visual Flight Rules (33%) and weather flights (22%) [4].

Cheung et al. investigated “whether there are consistent physiological changes” during false sensation of pitch in a simulator. After monitoring mean arterial pressure (MAP), heart rate (HR), Heart Rate Variability (HRV), respiration, Electrodermal Activity (EDA) and eye tracking, they reported changes in the cardiovascular responses such as MAP and HRV [5]. Another study on the influence of head position and eye state (open or close) on sympathetic activation during simulated type II SD found strong sympathetic outflow (increased HR and electrodermal response) on exposure to mild Coriolis illusion [6].

Lee et al. studied the pilot workload in a Boeing 747-400 flight simulator by comparing the physiological (heart rate) and the subjective measurement (NASA Task Load Index (NASA TLX) [7]. They found that the cardiac variables, incremental HR and HRV, besides NASA TLX, were sensitive indices for evaluating workload assessment during different phases of flight. They affirmed that NASA TLX is a practical tool for application in operational aviation environment. They also ascertained the usefulness of simulators to study cockpit workload [7].

An assessment of the mental workload, as a model of human-information processing, in the Instrument Meteorological Conditions (IMC), shall reflect the concurrent workload on the pilots during conditions conducive to disorientation. Therefore, an analysis of pilots’ psychophysiological and subjective measures was undertaken as a surrogate to study the mental workload during simulated IMC akin to inadvertent entry into clouds or flying on a dark night.

Methods

This case-crossover study, after institutional ethical approval [8], participants from amongst the male military pilots reporting for medical evaluation at the Institute of Aerospace Medicine (IAM), Bangalore, India. Some pilots with medical disabilities, who were fit for aviation duties, also participated 25 instrument rated pilots volunteered for this study.

The Airfox® DISO simulator was used to simulate experimental flights. The ‘free flight’ mode allowed changing the environmental conditions, simulating clouds, changes in ambient light conditions, runway dimensions etc. [9]. This ‘free flight’ mode was used to conduct the experiments in this study.

Physiological monitoring of the subjects was done with a proprietary wireless physiological monitoring system: Nexus-10® physiological monitoring and feedback platform (Mind Media B V, Roermond-Herten, Netherlands). This system allowed acquisition of signals from 0 to 800 Hz, including raw EEG, ECG, EMG, EOG etc. The acquired signals were wirelessly transmitted, using Bluetooth® wireless communication, for online monitoring and data storage. Online graphics presentation of the physiological parameters and retrieval of database, data processing, trend reporting, digital filtering and statistical functions were provided by a compatible software (BioTrace+ software®, Mind Media BV, Roermond-Herten, The Netherlands). Three physiological parameters viz., HR (from ECG), RR and GSR were recorded for this study.

NASA-TLX, a multi-dimensional subjective measure of mental workload assessment evaluates mental workload on six dimensions viz., mental demands, physical demands, temporal demands, own performance, effort and frustration. The first three dimensions assess the demands of the task imposed on the subject, while the latter three assess the interaction of the subject with the task [10]. NASA-TLX was used to assess the subjective assessment of mental workload in this study.

Results

The results were available for 21 of the 25 subjects. The mean age of the subjects (n=21) was 33.09 (±6.09) years. Their mean flying experience was 1775.33 Hours (±1297.38). Figure 1 presents the comparison of HR between the Day-Clouds (Control) and Evening-Night (Test) conditions. Evidently, the mean HR during the test conditions was higher, though not significantly, than the control (Table 1).

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Figure 1:

Comparison of Heart rate between the Day- Clouds (Control) and Evening-Night (Test) conditions

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Figure 2 presents the comparison of mean RR between the Day-Clouds (Control) and Evening- Night (Test) conditions. RR was found to be not significantly different both during control and test conditions (Table 2).

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Figure 2:

Comparison of respiration rate between the Day-Clouds (Control) and Evening-Night (Test) conditions

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Figure 3 presents the comparison of mean GSR between the Day-Clouds (Control) and Evening-Night (Test) conditions. CSR was found to be not significantly different both during control and test conditions (Table 3).

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Figure 3:

Comparison of ga vanic skin response betv 'een the Day-Clouds (Control) and Evening-Night (Test) conditions

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Table 4 presents the-comparison of NASA- TLX between the Day-Clouds (Control) and Evening-Night (Test) conditions. The subjective work load was found to be strongly significantly different between control and test conditions.

The correlation between psychophysiological measures and NASA TLX - Total workload during the Day-Clouds (Control) and Evening-Night (Test) conditions is presented at table V. HR showed mostly negative moderate correlation (0.3-0.5) to overall workload (and all the sub-scales, except effort score), during the test conditions. For the purpose of brevity, the correlation between psychophysiological measures and NASA TLX’s six sub-scales is not presented.

Discussion

An evaluation of the pilots’ mental workload during simulated IMC was undertaken in this study. The psychophysiological measures: HR, RR and GSR, were found to be not significantly different between control and test conditions (Table 1, 2, 3 & Figures 1, 2, 3).

Psychophysiological measures are indicators of the autonomic response to stress and the current level of alertness [5]. During simulated Coriolis cross coupling, Westmoreland et al. reported significant rise in HR and electrodermal response, but respiratory changes were non-significant [6]. Kallus et al. too reported marked increase in HR during BHA [13]. Similarly, Cheung et al. reported regular paced breathing rate of 12-15/min but a significant difference in mean HR (and MAP, HRV) and GSR between the control and experimental conditions, on exposure to simulated pitch illusion, with decrement in the flight performance [5]. The findings of this study are at variance with earlier studies, which could be due to variation in stimuli and collection methodologies [6]. particularly the choice of simulated conditions of comparing IMC at night as test conditions to the control conditions of intermittent IMC in clouds except during approach and landing.

Interestingly, Cheung stated that the results of HR measurement in Coriolis are controversial, with “very small, variable, or inconsistent changes” being reported [11]. But Lee et al. found HR to be the “most useful single variable for quantifying workload”, among the three variables studied viz., HR, HRV and skin conductance, during simulated landings [7]. It is known that the sinus arrhythmia of the respiratory cycle and phasic changes due to muscle tension and activity influence the cardiovascular system [5]. Thus the chosen variables in this study, especially HR, could have reflected changes in the respiration and striate muscle tension, if any, under the experimental conditions.

It is also known that “novel, unexpected, significant or aversive stimuli” elicit a specific or event-related skin conductance response [5]. Thus as “an indicator of psychological state or process of interest”, skin conductance (GSR/EDA) is considered as a direct representation of sympathetic activity [5]. One need not accept such indicators as ideal autonomic responses, since there is likely dissociation in the autonomic responses as per environmental emotional challenges, nature of the stimuli, and the perception of the individual about the extent and, thereby, subject’s response to the external situations faced [5]. Yet, such variables are useful for evolving an understanding about the pilot’s performance, including under IMC, as in the present study.

Strong significance was found between control and test conditions on NASA-TLX scores. This reflected the perceived difficulty of experimental conditions of flying at dusk and executing maneuvers at night, including ‘Black Hole approach’. Even though the subjects were on instruments during simulated EMC phase of flight, denial of external visual cues may have resulted in perceived difficulty of the task. In an experiment on steep approaches in low-visibility conditions, Boehm-Davis et al. reported an increase in subjective workload ratings (using Modified Cooper-Harper scale) with increasing difficulty associated with landing. Though not consistently significant in their experiment, they suggested that additional demands of steeper approaches and lowered ceiling gave rise to an increase in perceived workload [14].

Negative significant correlation between NASA TLX scores and HR during test conditions reflects that the subjects found the simulated test conditions to be severe as compared to the control conditions. It is conjectured that the negatively correlated changes in HR reflected the coping psychophysiological ability to the perceived workload. In this particular instance, both the psychophysiological and the subjective measure viz., HR and NASA-TLX are but two different measures of the mental workload, and there is no cause and effect relationship between the two [15]. On one hand, HR is an objective, real-time measure, which varies as per the task or phase of the flight. On the other hand, NASA-TLX provides the postrun subjective evaluation by the subject for the overall task performed. In addition, such a discrepancy could also be explained on the basis of the different mechanisms underlying the two responses - HR response to the varying workload throughout the experiment as compared to end-of- the-experiment feedback for the complete situation under evaluation. However, as compared to the subjective measures, the psychophysiological measures provide useful instantaneous and overall information [16].

The significant negative correlation between HR and NASA TLX highlights the importance of HR, as a single vital psychophysiological measure. This reinforces the assertion by Lindholm et al. that simulator based studies were useful to study cockpit workload, and that the HR was the “most useful single variable for quantifying workload” [17]. HR changes are parasympathetic manifestations of motor and perceptual preparatory responses and information processing activity. Thus cardiac activity is a useful measure of cortical activity to study cognitive processes [5]. Hence, while primary performance measure in aviation may not be easy to record without compromising the safety of the flight, psychophysiological measures, particularly HR provides a safe, non-intrusive, objective workload measure. Such psychophysiological variables also reflect moment to moment fluctuations in workload, without being affected by the subject’s judgement.

In comparison, subjective measures like NASA TLX may be affected by the subjects’ biases and retrospective ability to assess their own workload [18]. Yet, it is advocated that a combination of measures [19] must be utilized to study the mental workload, it being multi-dimensional for a single measure to adequately capture [20]. In addition, as Skinner et al. suggested, while addressing workload challenges in transition to 2- pilot tactical military airlift, that the psychophysiological measures can be gainfully used as debriefing tools to pinpoint episodes of significant workload [18]. This may have implications for ensuring aviation safety.

Limitations of the study

The small sample size for the study might have affected the outcome of this study as compared to others due to the variation in stimuli and collection methodologies [6]. Another limitation was non availability of HRV data for analysis, despite recording, due to limitations of the software. This denied the investigators a vital opportunity to study more variables than the ones reported in this study.

Conclusion

Twenty one male military pilots participated in this study to further an understanding about the mental workload during simulated SD susceptible in-flight conditions, using psychophysiological and subjective measures. The outcome of psychophysiological measures in isolation were not significant, though the subjective measures revealed the significant workload under test conditions during simulated TMC flight, akin to inadvertent entry into clouds or flying on a dark night. Significant negative correlation between HR and NASA TLX reinforced the importance of HR, as the most useful psychophysiological variable to quantify workload.