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Jan L. Souman, Alexander H. Wertheim, Ignace T.C. Hooge; Motion perception and localization during smooth pursuit eye movements. Journal of Vision 2003;3(9):540. doi: 10.1167/3.9.540.
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When a vertically moving dot is perceived during ocular pursuit of a horizontally moving pursuit target, it appears to move in a slanted instead of a vertical direction, which is thought to reflect incomplete compensation for the eye movement (Becklen, Wallach, & Nitzberg, 1984). We investigated the influence of three factors on this misperception: stimulus duration, pursuit velocity and the moment during pursuit at which the stimulus was presented.
While following a horizontally moving pursuit target with their eyes, participants were presented with a vertically moving dot (the stimulus), crossing the pursuit path. The task of the participants was to indicate either the perceived motion direction, or the (horizontal) position where the stimulus appeared or disappeared. Stimulus presentation duration varied from 200 ms to 1400 ms and the stimulus was presented half-way the pursuit, or shortly before or after this. In a second experiment, pursuit target velocity was varied from 6 /s to 14 /s.
Decreasing the stimulus presentation duration increased the perceived slant and reduced the horizontal distance between perceived begin and end points of the stimulus path. Perceived slant could be predicted fairly well from this horizontal distance, except for the shortest presentation durations. Increasing the pursuit velocity also caused the perceived slant to increase. The moment of stimulus presentation, however, did not have an effect on perceived slant.
An additional finding was that the whole slanted stimulus path was mislocated in the direction of the pursuit. This mislocalization did not depend on stimulus presentation duration or pursuit velocity, but it was larger when the stimulus presentation occurred earlier in the pursuit path.
Our results show that the perceived path of a vertically moving dot presented during horizontal ocular pursuit is not only slanted, but also displaced. This might have implications for theories of motion perception during ocular pursuit.
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