Shifts in listing's plane produced by vertical axis rotation: sustained ocular torsion due to semicircular canal stimulation.

PURPOSE

With the head upright and stationary, ocular torsion is confined by Listing's Law (LL), so that three-dimensional eye rotational axes form Listing's plane (LP). During head rotation, the vestibulo-ocular reflex violates LL by driving ocular torsion opposite to head torsion, sometimes out of LP. Saccades originating from non-Listing's initial torsional positions remain in a plane offset from, but parallel to, the original LP. The present study was conducted to determine whether whole-body yaw alters the position and orientation of LP.

METHODS

Eight normal subjects and six with unilateral vestibular deafferentation (UVD) underwent binocular eye and head movement recordings with 3-D magnetic search coils. Visual fixations were used to define LP, after which subjects underwent whole-body yaw rotation of 30 degrees or 70 degrees , at peak accelerations from 125 deg/s(2) to 2800 deg/s(2). Gaze during rotation was either central or 20 degrees up. After rotation, a dynamic LP (DLP) was defined during fixations.

RESULTS

Orientation and thickness of the DLP did not vary significantly from the previously defined LP; however, DLP was offset an average of 4 degrees +/- 4 degrees (mean +/- SD), 87% of head torsion relative to LP. Stimulus intensity, UVD, and starting vertical gaze direction had no effect on DLP offset or orientation. The DLP torsional offset declined toward the original LP with a time constant of approximately 1 minute, suggesting mediation by neural integration.

CONCLUSIONS

Yaw rotation can cause stable torsional offsets in the location of Listing's Plane.