Perception of the vertical with a head-fixed visual frame: Implications for using head-mounted displays

Head-mounted displays are now extensively developed and tested to be used in enhanced reality environments. The technique consists in transmitting synthetic visual information to the user's eyes in such a way that virtual objects are superimposed on the real world. Some displays give the feeling of viewing a rectangular head-fixed virtual screen with clearly visible contours. In addition, head-mounted displays allow for the presentation of head-fixed visual information to the user and this may be an advantage in some applications, military aeronautics for instance. Presenting head-fixed visual information may not be trivial with regard to actual models of spatial orientation. This study addressed the question of potential disorienting effects associated with head-mounted displays by investigating the influence of a head-fixed visual frame on the perception of the vertical when the head or the whole body was tilted in the frontal plane. In a first experiment, subjects were instructed to indicate the vertical by rotating a visual rod that appeared at the centre of the frame whilst tilting the head in various positions. This performance was compared with the effect on the subjective vertical of a tilted earth-based visual frame without head tilts as well as with the effect of tilting the head without a frame. With the tilted frames, subjects set the rod in an intermediate direction between the gravitational vertical and the orientation of the frame. Errors were substantially larger with a head-fixed visual frame during head tilt than with a tilted earth-based frame. This difference cannot be attributed to the addition of a postural effect caused by the head being tilted Moreover, continuous vision of the frame when its orientation changed improved performance only when the head and the frame were dissociated, i.e. with an earth-based frame. A second experiment investigated the effects of a head-fixed frame on the subjective vertical and on the voluntary control of head orientation when the whole body was tilted. The effect of a head-fixed frame was contrasted with the effect of a trunk-fixed frame. Results show that the head-fixed frame modified the head behaviour when subjects were instructed to align the head with the trunk. These errors contributed to an increase in the visual frame influence on the subjective vertical. Results of both experiments suggest that integrating visual information in the head-centric reference frame is crucial for spatial orientation. This Property of the perceptual system may be relevant for the design and use of head-mounted displays.