Investigating the Importance of Stereo Displays for Helicopter Landing Simulation

U.S. Air Force medical vision standards currently establish a minimum level of depth perception and ocular alignment. These standards apply not only to pilots but also to aircrew with scanner duty, which applies to aircrew involved in clearing aircraft for landing. This has been especially important since an accident in 1998 involving two H-60 aircraft that collided, where poor depth perception was identified as a contributing factor. However, similar standards do not apply for Army personnel in similar positions, and many other countries do not test for depth perception and ocular alignment even for pilots. Further, although much research has been conducted to examine the importance of stereo acuity and stereo displays in a wide variety of tasks, the results are mixed. The objective of the research presented here was to examine the effect of both stereo acuity and stereo displays on the performance of a helicopter landing task. For this research a representative task was selected in which subjects were required to discriminate the distance between the rear wheel of the aircraft and the top of an object over which the aircraft hovered. The simulation was constructed using the X-Plane software running on a pair of Windows PCs and viewed using a helmet mounted display. A unique aspect of this research is that observer stereo acuity, fusion range, and contrast sensitivity were thoroughly evaluated prior to participation. The results of the first evaluation indicated that observers with good stereo acuity scores performed significantly better on the operational task when stereoscopic video was used relative to monoscopic video. The results of the second evaluation indicated that operational performance could be predicted from a combination of vision test scores, but that stereo acuity test scores did not predict performance when used in isolation. INTRODUCTION A depth perception standard has been enforced for aviators since the early years of aviation. For example, Wilmer and Berens noted that “the value of stereoscopic vision....is of great value in judging distance and landing...The importance of this qualification seems to grow greater as our experience increases” [1]. Howard developed one of the first tests of depth perception for screening purposes and, on the basis of his research, believed that “to possess normal judgment of distance one’s binocular parallactic angle should not be greater than 8.0” [2]. However, the debate concerning the utility of depth perception has also been ongoing since the early 1900s. Howard (1919) noted “some examiners have questioned the absolute necessity of binocular single vision as a preliminary requirement.[2]” Although a 1996 Delta MD-88 crash at LaGuardia was partly attributed to defective stereopsis, some researchers have concluded that stereopsis is not required for flight safety, owing to the fact that other cues of depth are sufficient [3], [4]. According to U.S. Air Force (USAF) medical policy, good stereo acuity and ocular alignment are both considered to be critical for pilots, but also for nonpilot aircrew (Flying Class III (FCIII) aircrew) involved in certain tasks such as clearing aircraft for landing [5]. An FCIII depth perception standard has been enforced for USAF aircrew since 1998, following a fatal accident involving two H-60 helicopters where defective stereopsis was identified as a contributing factor [6]. However, a similar standard is not maintained for Army personnel in similar aircrew positions, and many other countries do not maintain a depth perception standard, even for pilots. IMAGE 2016 Conference Presented at the IMAGE 2016 Conference Dayton, Ohio – 28-29 June 2016 Cleared, 88PA, Case # 2016-2917. Research examining the importance of either stereo acuity or the use of stereo displays has had mixed results. In a systematic review of 71 experiments, previous researchers found that although about 67% showed a benefit of threedimensional (3D) displays, the remaining 33% either did not show a benefit or had very mixed results [7]. Similarly, a review of the importance of depth perception in aviation showed that not only is it difficult to clearly identify the importance of good stereo acuity, traditional methods used to measure stereo acuity may be lacking, which likely contributes to confusion concerning the utility of stereopsis and stereo displays [8]. In simulation and training applications, the use of stereo displays has been very limited. This may be due to several factors. Conventional knowledge has held that stereo is not useful beyond a few meters. Previous studies using electronic displays [9], [10] found stereo acuity thresholds of ~140 arcsec (i.e., many times higher than reported for real objects). Thus, previous experience with inadequate displays may have led to the conclusion that stereo cues would be ineffective for larger distances. Previous implementations of training systems with stereo displays proved difficult to implement [11], and two previous efforts to demonstrate the effectiveness of stereo displays for boom operator training were cancelled. Difficulties with the use of stereoscopic displays are well known and may be attributable to a number of different factors such as vergence-accommodation mismatch, image distortion/ misalignment between the left and right eye images, use of differing filters in the left/right eye (e.g., red/green filtering), conflicting depth cues (e.g., blur vs. disparity, lack of appropriate motion parallax), etc. [12]– [18]. As noted briefly above, a major limitation for many studies examining the utility of depth perception for performance of real-world tasks is that the measures of depth perception are often coarse and suffer from significant floor effects. If stereo acuity or other clinical metrics relevant to binocular health are actually obtained, they are often limited to, for example, a 40or 60-arcsec minimum threshold, or simply “fly positive,” meaning that subjects could see the 3D fly on a commonly available near stereo acuity test. Thus, part of the confusion concerning the utility of stereopsis may stem from the use of limited measures of binocular health. Although the potential limitations of some commonly used stereo acuity tests have been discussed [19]–[21], these tests are still frequently used. Our own research suggests that a more carefully designed computer-based stereo acuity test, although correlated with a more standard test, differs substantially (Figure 1) in outcome. As shown, there is a substantial floor effect on the standard test, and further, individuals obtaining the best score of 15 arcsec on the standard test may score anywhere from approximately 5 arcsec to 250 arcsec on the adaptive, threshold-based test. These results are consistent with previous research that suggests that the standard stereo acuity tests may actually test something other than stereo acuity. For this reason, we used our computer-based stereo acuity test in the research presented here rather than rely only on the more commonly available chart-based methods. Figure 1. Relationship between chart-based AFVT/AO Vectograph stereo acuity test and OBVA Lab, computerbased adaptive stereo acuity test. The objective of the research presented here was to examine the effect of both stereo acuity and stereo displays on the performance of a helicopter landing task. For this research a representative task was selected in which subjects were required to discriminate the distance between the rear wheel of the aircraft and the top of an object over which the aircraft hovered. To initiate this line of research, we have broken down a very complex call-to-landing task into subcomponents, beginning with the hover task presented here. Future research will examine time to contact and height esimtation prior to researching performance in a full combat landing simulation. The simulation was constructed using X-Plane software running on a pair of Windows PCs and viewed using a helmet-mounted display (HMD). A unique aspect of this research is that each observer’s stereo acuity, fusion range, and contrast sensitivity were thoroughly evaluated using computer-based vision tests developed in our laboratory prior to participation in the simulated helicopter landing task. It is important to note that depth perception involves much more than binocular disparity. A wide variety of monocular cues, such as optic flow [22], [23], motion parallax [16], [17], [24], relative size, and occlusion [25], all contribute to depth perception. In this research, the use of a head-tracked, wide field of view HMD IMAGE 2016 Conference Presented at the IMAGE 2016 Conference Dayton, Ohio – 28-29 June 2016 Cleared, 88PA, Case # 2016-2917. and highly detailed simulated environment preserved many of the cues to depth that would normally be encountered in a natural environment. Thus, the research described here should be relevant for examining the contribution of stereo displays and quality of vision to the performance of a highly complex task such as a helicopter call-to-landing.