Convergence during human vertical saccades: probable causes and perceptual consequences.

1. When a downward saccade is made between equidistant targets, convergence consistently occurs during the saccade: about 1 deg overconvergence after an 8 deg saccade, with either binocular or monocular viewing, with either far (3 m) or near (30 cm) viewing distance. 2. During binocular viewing, this unnecessary convergence is corrected by divergence movement with a half-time of about 200 ms. During monocular viewing of far targets, similar post-saccadic divergence occurs, but for monocularly-seen near targets, recovery is considerably slower. 3. Vergence changes associated with upward saccades are much smaller and typically more variable among subjects. 4. The up-down asymmetry of intrasaccadic vergence changes can be accounted for by superposition of two plausible adventitious processes: co-contraction of the vertical recti, and tension increase (upward saccades) or tension release (downward saccades) in the superior oblique muscles. 5. During the 1000 ms after an upward saccade, constriction of the pupil consistently occurs; it apparently represents a near-triad response, for which concurrent convergence is masked. Such near-triad activation during upward gaze would presumably be necessary to counterbalance residual steady-state torques from the superior oblique muscles. 6. The up-down asymmetry of intrasaccadic vergence changes also arises when targets require both a vergence change and a vertical shift of gaze, thereby accelerating refixation for the typical natural spatial configuration, in which nearer objects are lower in the visual field. 7. During binocular viewing of equidistant targets, the convergence resulting from downward saccades produces large transient disparities, which can be expected to lead to biased evaluations of relative distances to targets. Several up-down illusions involving apparent distance may well be due to these disparities, including (a) backward tilt of the apparent vertical and of the vertical horopter, (b) the 'soup-bowl sky' illusion, and (c) the 'diverging sunbeams' illusion.