Variability of the saccadic main sequence within and between tasks

The kinematics of saccades are classically described by the main sequence, a robust relationship linking saccade peak velocity or duration to saccade amplitude. This relationship is consistent in its form across individuals but the model parameters that characterize it vary. While most studies agree that the main sequence shows good test-retest stability and internal consistency, evidence regarding the influence of task demands is less consistent. In the present study, we examined task-related variability and within-task variability in the main sequence across a large sample of 62 participants, based on two eye-tracking experiments including visually guided and free-viewing saccades. We analysed the main sequence mathematical models both between tasks and within each task. Our results show that, although the overall shape of the main sequence remains stable, the associated model parameters vary significantly as a function of task. Notably, task-related effects exceed within-task variability, revealing subtle yet robust influences of task on saccadic kinematics. These findings underscore the importance of accounting for task context when applying the main sequence in research or clinical settings and highlight the added value of large-sample studies for disentangling sources of variability in oculomotor behaviour.