Predicted Position Error Triggers Catch-Up Saccades During Sustained Smooth Pursuit

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Nachmani, Omri
Saccades , Smooth-Pursuit , Eye-movement , Prediction , Motor control , Uncertainty
Humans are largely visual animals and require that images of objects of interest must be held steadily on the fovea, a region on the retina that allows for high visual acuity. In order to maintain gaze on objects in the environments, humans must move their eyes to keep the object centered on the fovea using saccades, rapid eye movements that redirect the line of sight to a peripheral target, and smooth pursuit, which allow for tracking of low-velocity objects moving within the environment. When objects are fast or unpredictable, the brain coordinates the execution of both pursuit and saccades to catch-up to the target. Deciding to trigger a catch-up saccade during pursuit influences the quality of visual input. This unconscious decision is a trade-off between tolerating being off target when no saccade is triggered or a transient loss of vision during the saccade as the eye moves rapidly. Although catch-up saccades have been extensively investigated, it remains unclear how the trigger decision is made by the brain. de Brouwer et al (2002) demonstrated that catch-up saccades were less likely to occur when the expected time to contact a target using pursuit alone is between 40 and 180ms into the future, referred to as the smooth zone. More recently, we proposed a model (Coutinho et al., 2018) that relies on a probabilistic estimation of predicted future position error (PEpred). Informed by model predictions, we hypothesized that saccade trigger time length and variability will increase when pre-saccadic predicted errors are small or visual uncertainty is high (using a blurred target). Data collected from human participants performing a double step-ramp task showed that large pre-saccadic PEpred (>10deg) produced fast reacting saccades regardless of the level of uncertainty while saccade trigger times preceded by small PEpred (<10deg) were significantly modulated by high uncertainty. The data supports our hypothesized role of PEpred in deciding when to trigger a catch-up saccade during smooth pursuit and indicates a common predictive decision process that governs motor coordination of eye movements.
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