FIG. 1.

Semicircular canal (SCC) coordinate system used for the description of head rotation and three-dimensional vestibulo-ocular reflex (3D VOR) eye rotation responses. Skull coordinates +X, +Y, and +Z are mutually orthogonal stereotaxic axes perpendicular to the stereotactic coronal, sagittal, and horizontal planes, respectively. +X is nasal, +Y is out the left ear canal along the interaural axis, the plane containing +X and +Y passes through the inferior-most point of the cephalic edge of each orbital rim, and +Z is superior. SCC coordinate system axes +LARP, +RALP, and +H used in this study approximate the mean SCC axes measured from CT reconstructions of the animals used in this study. They are mutually orthogonal and centered on the XYZ stereotaxic origin. The mean +H axis is close to +Z but pitched back from the stereotaxic +Z axis by 15 ° toward the occiput. The +LARP and +RALP axes are each 45 ° off the midsagittal plane, and 45 ° off the +Y axis, but pitched 15 ° up from the [X, Y, Z] = [−1, 1, 0] and [1, 1, 0], respectively. Eye rotation velocity polarities are expressed in this canal-based reference frame using a right-hand rule convention, so that positive eye rotations about +LARP pitch the eye upward while rolling it clockwise (as viewed from behind the head); positive RALP eye rotations pitch the eye downward while rolling it clockwise, and positive H eye rotations yaw the eye to move the pupil toward the left ear.