Our finding for saccades is in contrast to our results for smooth pursuit, where mean eye distance to the target increased with target size across seven out of eight participants in Experiments 1 and 2 (
Figure 5), and for the two participants tested with a larger range of sizes in Experiment 4. The data suggests that unlike saccades, eye placement in smooth pursuit is not simply a consistent offset, but is related to target size. This noncentering tendency is also consistent with a lack of foveation of the pursuit target on some of the trials.
These findings may reflect the everyday purpose of the two eye movement types. In the case of saccades, eye movements are initiated to bring the region of highest visual acuity onto a feature of interest in the environment for localization or identification. In the case of smooth pursuit, the main goal is to maintain an object in the visual field of the observer (Blohm et al.,
2005; de Xivry & Lefevre,
2007), while the specific task at hand and the nature of the target, may or may not require the high acuity of the fovea. Given a specific task, the two types of eye movement can be highly synergistic with each other, consistent with the tightly linked underlying physiology subserving the two (de Xivry & Lefevre,
2007; Orban de Xivry, Bennett, Lefèvre, & Barnes,
2006). However, when the task does not require identification of a specific feature, foveation is likely unnecessary. Catch-up saccades may occur in cases where distinct features of interest need to be identified (Heinen et al.,
2016a), if there is an attentional component (Heinen, Potapchuk, & Watamaniuk,
2016b), or if retinal slip causes the object to disappear from the observer's visual field. Furthermore, catch-up saccades may be required during pursuit of fast-moving or unpredictable targets, where the smooth pursuit system is unable to adjust eye velocity such that position error accumulates (de Xivry & Lefevre,
2007).