Description of Visual-Inertial Phase Angles

The research described below manipulates the phase angle between the visual and inertial motions in order to produce conflicting self-motion cues. A confusing point about visual-inertial phase angles is that, ordinarily, visual and inertial motion cues are 180$^{\circ}$ out of phase (see Figure 3.1, on page ). For instance, when one turns to the right, the visual field flows to the left. More explicitly, if one stares perpendicular to one's shoulders while turning to the right, after having turned, a point which one was initially observing will have moved to one's left.

Thus, the ``ordinary'' situation is for inertial and visual stimuli to be 180$^{\circ}$ out of phase. The ``most contradictory'' condition occurs when the inertial and visual stimuli are exactly in phase. For instance, if the visual background moves to one's right, one tends to feel that one is moving to the left. If the inertial motion is also to the right, then one has completely conflicting motion cues.

It is confusing to use the term ``in phase'' to describe inertial and visual stimuli which are completely conflicting in terms of their effect on self-motion perception. To avoid this problem, the phase angles in this chapter are given between the vection and inertial curves rather than between the visual and inertial curves. That is, a phase angle of zero implies a consistent sense of self-motion from the two kinds of stimuli. The vection curve differs from the visual curve by 180$^{\circ}$. A phase angle of $+90^{\circ}$ is defined here to imply that one's inertial sense of self-motion is 90$^{\circ}$ ahead of one's visual sense of self-motion. Conversely, a phase angle of $-90^{\circ}$ implies that one's inertial sense of self-motion is $90^{\circ}$ behind one's visual sense of self-motion.

The word ``consistent'' is used to indicate a vection-inertial phase angle of zero, and ``conflicting'' to indicate all other vection-inertial phase angles.

Human Interface Technology Lab