Modulation of saccade curvature by ocular counterroll.

PURPOSE On close inspection, it can be seen that most saccadic trajectories are not straight but curve slightly; in other words, they are not single-axis ocular rotations. The authors asked whether saccade curvatures are systematically influenced by static ocular counterroll (OCR). METHODS OCR was elicited by static whole-body roll position. Eight healthy human subjects performed horizontal and vertical saccades (10 degrees amplitude; 0 degrees and 10 degrees eccentricity; head-fixed coordinate system) in upright and ear-down whole-body roll positions (45 degrees right, 45 degrees left). Three-dimensional eye movements were recorded with modified dual-search coils at 1000 Hz. RESULTS Saccade curvature was systematically modulated by OCR depending on saccade direction. In the horizontal-vertical plane, primarily vertical saccades were modulated with downward saccades curving toward the upper ear and upward saccades curving toward the lower ear. Modulation of saccade curvature in the torsional direction correlated significantly with OCR only in abducting saccades. CONCLUSIONS No universal mechanism, such as visual-motor coordinate transformation or kinematic characteristics of the saccadic burst generator, alone could explain the complex modulation pattern of saccade curvature. OCR-induced changes of the ocular motor plant, including transient force imbalances between agonist eye muscles (vertical rectus and oblique muscles) and shifting eye muscle pulleys, are suitable to explain the found direction-dependent modulation pattern.