When wearing a monocular head-mounted display (HMD), one eye views the HMD symbology while both eyes view
an out-the-window scene. This may create interocular differences in image characteristics that could disrupt binocular
vision by provoking visual suppression, thus reducing visibility of the background scene, monocular symbology, or
both. However, binocular fusion of the background scene may mitigate against the occurrence of visual suppression, a
hypothesis that was investigated in the present study. Observers simultaneously viewed a static background scene and
HMD symbology while performing a target recognition task under several viewing conditions. In a simulated HMD
condition observers binocularly viewed a background scene with monocular symbology superimposed. In another
condition, viewing was dichoptic (i.e. completely different images were presented to the left and right eyes).
Additionally, one control condition was implemented for comparison. The results indicate that for continuously
presented targets binocular rivalry did not have significant effects on target visibility. However, for briefly presented
targets, binocular rivalry was shown to increase thresholds for target recognition time in HMD and dichoptic viewing
conditions relative to the control. Impairment was less in the HMD condition. Thus, binocular fusion of a background
scene can partially mitigate against the occurrence of visual suppression. However, some suppression still exists which
occurs between monocular pathways. Implications for the integration of monocular HMDs into Air Force training
environments will be discussed.
The Joint Helmet Mounted Cueing System (JHMCS),is being considered for integration into the F-15, F-16, and F-18 aircraft. If this integration occurs, similar monocular head-mounted displays (HMDs) will need to be integrated with existing out-the-window simulator systems for training purposes. One such system is the Mobile Modular Display for Advanced Research and Training (M2DART), which is constructed with flat-panel rear-projection screens around a nominal eye-point. Because the panels are flat, the distance from the eye point to the display screen varies depending upon the location on the screen to which the observer is directing fixation. Variation in focal distance may create visibility problems for either the HMD symbology or the out-the-window imagery presented on the simulator rear-projection display screen because observers may not be able to focus both sets of images simultaneously. The extent to which blurring occurs will depend upon the difference between the focal planes of the simulator display and HMD as well as the depth of focus of the observer. In our psychophysical study, we investigated whether significant blurring occurs as a result of such differences in focal distances and established an optimal focal distance for an HMD which would minimize blurring for a range of focal distances representative of the M2DART. Our data suggest that blurring of symbology due to differing focal planes is not a significant issue within the range of distances tested and that the optimal focal distance for an HMD is the optical midpoint between the near and far rear-projection screen distances.
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