Modulation of saccade curvature by ocular counterroll

Abstract

PURPOSE: On close inspection, most saccadic trajectories are not straight, but curve slightly, i.e. they are not single-axis ocular rotations. We 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 (amplitude: 10 degrees ; eccentricity: 0 degrees and 10 degrees ; 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 1000Hz. 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 towards the upper ear and upward saccades curving towards the lower ear. Modulation of saccade curvature in the torsional direction only correlated significantly with OCR in abducting saccades. CONCLUSIONS: No universal mechanism like visual-motor coordinate transformation or kinematical 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.