Walsh & Hoyt: Three Dimensions of Eye Motion

The eyeball and the head undergo rotation with three degrees of freedom: rotation about the yaw axis is horizontal, about the pitch (interaural) axis is vertical, and about the roll (naso-occipital) axis is torsional. In theory, the eye could assume an infinite number of torsional positions for any gaze direction. However, during fixation, saccades, and smooth pursuit movements, eye rotation has only two degrees of freedom, with torsion being constrained. In other words, there is only one fixed torsional eye position for each combination of horizontal and vertical eye position. This constraint on torsion is mediated by neural innervation, by the mechanical properties of the orbital tissue, or a combination of both. Donders first described this phenomenon by observing the rotation of his own eye about the visual axis using afterimages. Listing quantitatively defined the amount of torsion for each gaze direction: any position of the eyeball can be reached from primary position by rotation about an axis that is perpendicular to the line of sight in primary position. The collection of these axes for all rotations that start from primary position constitutes Listings plane. Donders and Listings observations have been heralded as ""laws."" Listings law specifies the constraint on torsion expressed by Donders law. One consequence of Listings and Donders laws is that motor efficiency is optimized. By ensuring that the eye stays near the center of its torsional range, the laws permit quick responses to unpredictable targets that may appear from any direction.