Why do patients with impaired vergence not show "saccadic" vergence?

Low gain of the smooth-pursuit system causes correcting saccades that are clinically observed as "saccadic pursuit." In contrast, low gain of the vergence system during tracking does not appear to cause "saccadic tracking." To further evaluate whether this clinical observation is caused by a missing interaction of vergence to step and ramp targets, vergence eye movements to step-ramp, step, and ramp targets in 3-dimensional space were tested. Results show that vergence to a step and a ramp stimulus is additive in the step-ramp paradigm, indicating no interaction of both components.