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Alexander Goettker, Doris Braun, Alexander Schütz, Karl Gegenfurtner; Saccadic eye movements affect perceived speed. Journal of Vision 2017;17(10):1160. doi: 10.1167/17.10.1160.
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Smooth pursuit and saccadic eye movements are used in combination to track moving targets. Pursuit is mainly driven by the velocity of the target and saccades by its position, even though interactions between these input signals have been shown. We wanted to see whether different oculomotor responses during the tracking of a moving object lead to changes in perceived speed. In a speed discrimination task observers had to judge the speed of the pursuit target in comparison to a memorized standard stimulus. We manipulated the eye crossing time in a classical Rashbass step-ramp paradigm to elicit different eye movement responses (pure pursuit, backward saccades, forward saccades) to targets moving at the same physical speed. Both eye movements, pursuit and additional catch-up saccades, used for tracking a moving target adjusted their responses based on the position and speed of the target. The pursuit system adjusted the speed of the eye to account for the target position, but interestingly, only the occurrence and direction of additional saccadic eye movements was related to the perceptual judgments of the participants. Trials with forward saccades were perceived as faster than trials with backward saccades. We conducted an additional fixation control condition and found that the same physical targets led to smaller differences in perceived speed than trials with saccadic eye movements during tracking. Previous results have shown interactions between smooth pursuit and saccades, and interactions in the use of position and velocity in the control of these eye movements. Our results show that the execution of catch-up saccades can also affect the perceived speed of the tracked target. Saccades could be used as evidence for an update of an internal representation of target speed or the high eye speed during saccades biases the extraretinal signal used to account for induced motion during tracking.
Meeting abstract presented at VSS 2017
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