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Among the motion configurations tested, it was the translational motion that always had a strong effect. Yaw rotational motion was shown to be a redundant cue, and less roll motion was more acceptable (when comparing percent of full motion) than less lateral translational motion. Requiring less motion in the rotational axes is still consistent with the revised set of the Sinacori criteria that were suggested in this report. A question arises as to why only one of the three rotational cues is redundant. That is, why is yaw rota-tional motion redundant, but pitch and roll motion useful? A possibility is that the pitch and roll rotational cues (as sensed by the inner ear) are no more important than the yaw rotational cue, yet their usefulness arises from the additional cues concomitant with pitch and roll motion. These additional cues have two sources. First, pitch and roll motion cues interact with the gravity vector, as discussed in section 7. Very few simulators can remove the specific force cue that arises from either pitch or roll attitude. As such, evaluating the effect of the angular cue only is challenging; it has only been investi-gated by Jex et al. (ref. 41) in which subjects rolled while lying on their backs (thus the gravity vector did not change relative orientation during an orientation). Although that study showed an effect of roll, this could be due to another factor (in addition, it might be argued that compelling roll visual cues were not present in that study, as only a horizon line was present). The other factor is the tangential acceleration that results from the moment arm between the roll center of rotation and the motion sensors on a human. It is not possible to eliminate the effect of these tangential accelerations completely, for the human motion sensors are in different locations (inner ear, neck, buttocks, limbs). Some experiments have isolated the head by fixing it in an apparatus and subsequently performing reorientations about that axis, but those were not piloted experiments. Thus, when pitching and rolling, isolating the angular cue from the translational cue is difficult if not impossible. It is only in yaw that many of these cross-coupling effects into the translational axes are lessened (but perhaps not removed completely, as discussed in sec. 3). Thus, the above reasons may explain why the requirements on the yaw axis are different from pitch and roll. Although the longitudinal axis was not examined in these studies, no reason is offered as to why the requirements in that axis might be different from those in either vertical or lateral translation.
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