Three-dimensional human eye movements are organized differently for the different oculomotor subsystems

Three-dimensional (3-D) eye movements during steady fixation, saccades and smooth pursuit are governed by a powerful strategy restricting eye position to 2 degrees of freedom by keeping the torsional component close to o (Listing's law). To test if Listing's law is also obeyed during optokinetic and vestibular stimulation, the study compares 3-D eye movements in humans during pursuit, optokinetic and vestibular activation using similar gaze trajectories. The optokinetic and vestibuloocular reflexes deviate from Listing's law showing a strategy lying about halfway between optimal full-field retinal image stabilization and Listing's law. The fact that for similar gaze trajectories the brain can choose between different 3-D movement strategies depending on the oculomotor subsystem currently in action (supposedly to optimize for the specific requirements of a particular oculomotor subsystem) suggests that Listing's law is due to a neurally-im-posed constraint on the oculomotor output.