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Rick H. Cai; Compression of perceived motion trajectories. Journal of Vision 2005;5(8):653. doi: 10.1167/5.8.653.
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In this study, we investigated whether the perceived length of a motion trajectory is equal to its veridical length. A bar moved on a computer screen along a straight path. One segment of the trajectory was marked out by turning the bar into a different color. The length of this marked segment was compared with that of a comparison stimulus displayed below the motion trajectory, which consisted of the very same motion segment but displayed statically (i.e. in constant view as a rectangle). The length of this comparison stimulus thus revealed the veridical length of the marked segment. It was found that the marked segment drawn out by the moving bar was perceived as shorter than the static comparison segment. This shortening was not caused by the color difference per se. Marking the segment by a different bar size produced the same results. Nor was it caused by the marker onset. A segment produced by the offset of the moving bar (i.e. a gap inserted into the motion trajectory) was also perceived as shorter. Prior research on the ‘flash-lag illusion’ has shown that a moving stimulus is perceived as located ahead of, not behind, its veridical location. The current finding thus poses a dilemma: if the motion trajectory is perceived as shorter than its real length, how can the moving stimulus be perceived as located ahead of its veridical position? Additional experiments explore this dilemma in the context of possible differences in the neural representation for continuous and discrete events.
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