On Pulfrich-illusion eye movements and accommodation vergence during visual pursuit

When the Pulfrich illusion is perceived with stationary fixation, and visual pursuit of the pendulum is then initiated, rapid vergence changes occur which correspond to the illusory elliptical path. During steady-state visual tracking of the illusion, however, the eyes move along a planar path without systematic changes in vergence. These latter pursuit movements with monocular filter involve large fixation disparities relative to unobstructed vision (0.5 degree to 1 degree divergence); hence, it is proposed that the planar tracking path probably results from strong dominance of the oculomotor system by stimuli from the unobstructed eye. During visual tracking with monocular filter and a target moving along a nonillusory elliptical path in depth, appropriate changes in vergence occur, but comparable vergence changes also arise when the target is fully hidden from one eye. This response apparently represents a superposition of accommodation vergence upon smooth pursuit movements; similar responses also occur during monocular tracking of a target moving around a circular path in depth.