The remarkable saccades of asymmetrical vergence

The saccades that usually arise near the onset of asymmetrical changes in vergence, when one eye is aligned with both targets, are remarkably different from ordinary saccades: (1) the excursions of the two eyes are typically very unequal, often differing by several fold from each other; (2) mean excursion (version) is extremely variable across replicate tests with identical targets; (3) at the end of the saccades, eye orientation is usually not even briefly stable: the aligned eye immediately reverses its movement, indicating that the pulse in muscular forces is apparently not followed by a corresponding step; and (4) a second saccade in the opposite direction can immediately follow the initial saccade of asymmetrical divergence, with no sign of refractoriness. These phenomena suggest that the pulse and step components of saccadic motoneuron activity may be generated by largely independent processes; that the step component for each eye depends only on that eye's visual input; and that the pulse components generated for each eye depend on weighted averaging of visual stimuli that impinge on both eyes. This interpretation is incompatible with most current models of saccade generation, but was anticipated in its essentials by Ditchburn [(1973) Eye movements and visual perception. Oxford: Clarendon Press]. A corollary of this hypothesis is that disparity-evoked vergence changes can be viewed as the general-case output from that system which produces fully conjugate saccades as a special case.