We have not yet considered the effect of each of these six cases in terms of their mismatch with vestibular cues. If the user is not moving relative to the Earth, then only gravity should be sensed by the vestibular organ (in particular, the otolith organs). Suppose the user is facing forward without any tilt. In this case, any perceived acceleration from vection would cause a mismatch. For example, yaw vection should cause a perceived constant acceleration toward the rotation center (recall Figure 8.2(b)), which mismatches the vestibular gravity cue. As another example, downward vertical vection should cause the user to feel like he is falling, but the vestibular cue would indicate otherwise.
For cases of yaw, pitch, and roll vection at constant angular velocity, there may not be a conflict with rotation sensed by the vestibular organ because the semicircular canals measure angular accelerations. Thus, the angular velocity of the viewpoint must change to cause mismatch with this part of the vestibular system. Sickness may nevertheless arise due to mismatch of perceived linear accelerations, as sensed by the otolith organs.
If the head is actually moving, then the vestibular organ is stimulated. This case is more complicated to understand because vestibular cues that correspond to linear and angular accelerations in the real world are combined with visual cues that indicate different accelerations. In some cases, these cues may be more consistent, and in other cases, they may diverge further.
Steven M LaValle 2019-03-14