Rolling motion makes the eyes roll: torsion during smooth pursuit eye movements
26.453, Saturday, May 11, 2:45 - 6:45 pm, Orchid Ballroom
Janick Edinger1, Dinesh Pai2,3, Miriam Spering1,3; 1Ophthalmology & Visual Sciences, University of British Columbia, 2Computer Science, University of British Columbia, 3Brain Research Centre, University of British Columbia
Introduction: We frequently observe horizontal and vertical movements of the eyes, but what is less often noted is the fact that the eye can also rotate about the line of sight, resulting in torsional eye movements. Torsion may serve to compensate for rotations of the head, but its exact function is unclear. Here we examine the functional role of torsion during smooth pursuit eye movements by testing whether torsion can be visually triggered. Methods: Observers (n=8) tracked a random-dot pattern, which moved to the left or right at 10 deg/s, and rotated around its center, either clockwise or counter-clockwise relative to translational motion, at speeds ranging from 151-208 rad/s. In control experiments, we varied stimulus size (4-12 deg) and elevation of gaze. We recorded 3D eye position with a head-mounted Chronos ETD in head-fixed observers. Results: We discovered strong torsion in the direction of stimulus rotation during smooth pursuit. Torsion was fastest in response to natural rotation, the direction an object would rotate if it was rolling on the ground. Natural and unnatural rotation triggered two different patterns of torsion: natural rotation resulted in smooth, continuous torsion at a significantly higher rotational speed than unnatural rotation, which triggered a torsional nystagmus. Natural rotation also produced more accurate pursuit. Torsion increased as a function of stimulus size, but effects were constant across gaze elevations. Conclusion: We provide the first evidence of visually-triggered torsion during pursuit. The torsional strength varied systematically with visual stimulus properties such as direction and size, indicating that torsion could play an important role in stabilizing pursuit during image rotation. Listings Law, which describes the kinematics of 3D eye movements and predicts zero torsion during pursuit, does not hold here.