Listing's law: clinical significance and implications for neural control.

Listing's law governs the three-dimensional orientation of the eye and its axes of rotation. It states that, when the head is fixed, there is an eye position called primary position, such that the eye assumes only those orientations that can be reached from primary position by a single rotation about an axis in a plane called Listing's plane. Listing's law can also be expressed in terms of any initial eye position, not just primary position (Listing's half-angle rule). The binocular extension of Listing's law is equivalent to Listing's law when the vergence angle is zero, and adjusts the eyes' torsion when they converge. Listing's law holds during fixation, saccades, smooth pursuit, and vergence, but not during sleep and vestibulo-ocular reflex, suggesting that it is actively implemented by a neural mechanism. Orbital constraints, such as "pulleys," may also play a role. Adherence to Listing's law and its extension may serve the purpose of optimizing motor efficiency, or simplifying neural processing for binocular vision, or both. The study of Listing's law not only allows us to understand the organization of neural and mechanical factors in the control of three-dimensional eye movements, it also has important clinical implications for strabismus and the optimal management of this disorder.