Mechanical interdependence of version and vergence eye movements

In this paper, the authors investigate whether the idea of independent control of version and vergence eye movements is compatible with the mechanics of the eye plant. By computing the change in the axes of action of the eye muscles as a function of ocular vergence, they prove that, regardless of the muscle pulley locations, the required muscle activity for vertical version depends on the initial vergence angle. The binocular extension of Listing’s law (‘L2’) describes how the torsional orientation of the eye depends on both gaze direction and ocular convergence. The authors show that for each vergence angle there is a range of possible muscle pulley locations that would cause independent control of version and vergence to result in L2. They also show that this mechanical explanation of L2 requires that the muscle pulleys move as a function of vergence.