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Scott Watamaniuk, Stephen Heinen; Fixation and pursuit show similar behavior with complex stimuli. Journal of Vision 2017;17(10):887. doi: 10.1167/17.10.887.
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During smooth pursuit, retinal image motion helps stabilize a target by modulating eye velocity. We previously showed that increasing the number of elements comprising a pursuit target increases open-loop eye acceleration (Heinen et al., 2016). We hypothesized that pursuit was facilitated because more stimulus elements created a stronger motion signal. The purpose of fixation appears similar to that of pursuit – to stabilize a visual target. If true, the fixation system might also utilize a motion signal to stabilize gaze since during fixation, the eyes continuously create retinal motion as they drift across a target. To test this, we asked if eye velocity during fixation is altered when the number of stimulus elements in a fixation target is changed. Observers fixated either a small spot target, a 6° circular array of 8 dots, or a 9-dot conglomerate of these stimuli. Eye movements were measured using an EyeLink 1000 eye tracker. We found that smooth eye velocity during fixation decreased when the number of elements in the stimulus increased, evidence that retinal motion facilitated fixation as it does open-loop pursuit acceleration. During pursuit, catch-up saccades are not linearly related to the number of stimulus elements, rather they occur more frequently when a stimulus has a central element (Heinen et al., 2016). To see if the fixation/pursuit analogy extends to saccadic intrusions, we also looked at microsaccades rate for the different stimulus configurations. Like catch-up saccades during pursuit, during fixation microsaccades were also more prevalent when there was a central element. The results suggest that the fixation system uses a motion signal to reduce slow drifts and increase image stability, and might do so using retinal-motion feedback. Moreover, small central elements encourage foveation during fixation as they do during pursuit.
Meeting abstract presented at VSS 2017
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