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Alex Holcombe, Daniel Linares; Dissociating motion-induced position illusions by the velocity dependence of both their magnitude and their variability. Journal of Vision 2009;9(8):1093. doi: 10.1167/9.8.1093.
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Observers mislocalize a moving object's color change in the direction of its motion (Cai & Schlag 2001). One theory of this Cai effect and of the related flash-lag effect is that motion signals shift perceived position to help overcome neural latency. Others suggest perception of a transient change is delayed relative to a moving object or that the transient triggers a time-consuming process. All these theories predict that the illusory displacement should increase with speed. For the Cai effect, we used a blob orbiting fixation that suddenly changed color and asked observers to report the perceived position of the change. Contrary to the predictions, spatial shifts were nearly identical across a five-fold range of speeds. With the same stimulus configuration, we measured the flash-lag effect (by asking observers to report the position of the blob when the fixation mark changed color) and Frohlich effect (by asking observers to report the position at which the blob first appeared). Results for the Frohlich effect were similar to the Cai effect results, whereas the flash-lag effect was larger and showed a significant increase with speed. The effects also differed in their variability from trial to trial: the variability of the Cai and Frohlich shifts did not increase with speed, while the flash-lag variability increased steeply with speed. These results suggest the involvement of a process with low temporal resolution unique to the flash-lag task, perhaps one that binds the flash (here, a sudden color change) with the moving object. The Cai and Frohlich effects appear to be caused by a distinct spatial shifting mechanism, one not very useful for overcoming neural delays as it does not scale with speed. Position judgments in the flash-lag task may reflect a combination of this spatial shifting with coarse temporal binding.
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