This suggestion was investigated using an eye muscle model exhibiting the important nonlinearities of the static oculomotor plant. The control parameters are approximately rotation vector components. In this model Listings law at tertiary positions is a consequence of separable control. When a simple additive controller based on retinal location is used, fixation errors are small over a wide field of view, and no extra errors are incurred by the mechanical implementation over and above inevitable geometric errors.
Binocular control separates naturally into version and vergence components.
If the vertical command is kept constant and equal for the two eyes, they remain in vertical alignment during vergence and version movements. These findings indicate that the forward kinematics of the extraocular muscle system is unexpectedly simple. This may have significant consequences for both the development and calibration of fixation control. For example, the simplification in the control of binocular alignment may be particularly important developmentally in view of the relationship of amblyopia to strabismus.