The oculomotor plant and its role in three-dimensional eye orientation

Following years of controversy, a combined approach consisting of both single unit recordings from cyclovertical extraocular motoneurons and electrical microstimulation of the abducens nerve have shown that, for visuallyguided eye movements, motoneuron commands balance out to appear two-dimensional (2D; i.e. horizontal/vertical). These studies have also shown that the mechanical properties of the eyeball (perhaps mediated by extraocular muscle pulleys) provide for an eye position-dependent muscle pulling direction, thus accounting for the 'half-angle rule', a property of three-dimensional (3D) eye rotations necessary to keep the eyes in Listing's plane during visually-guided eye movements. Yet, how eye movements that do not follow Listing's law (e.g. the rotational vestibulo-ocular reflex, VOR) use an eye plant that implements the half-angle rule remains a challenge for future studies.