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Sirui Liu, Peter U. Tse, Patrick Cavanagh; Distance not time imposes limits on accumulation of illusory position shifts in the double-drift stimulus. Journal of Vision 2019;19(10):288. doi: https://doi.org/10.1167/19.10.288.
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When the internal texture of a Gabor patch drifts orthogonally to its physical path, its perceived motion deviates dramatically from its physical path. The local position shifts orthogonal to the motion path accumulate to create the deviation so that, for example, a 45° oblique physical path will appear to be vertical. However, at some point, a limit is reached where the path resets back to its physical location and a new accumulation starts, making a new perceived vertical segment parallel to the first but offset horizontally. This reset can be forced by introducing a temporal gap in the path and the perceived offset reveals the magnitude of the accumulation up to the reset. If the accumulation has been linear and free of loss during the first segment, it must 70.7% (=1√2) of the physical path length for the 45° angle of the illusory deviation (Lisi & Cavanagh, 2015). Anything less than 70.7% indicates that some reset or saturation has occurred before the temporal gap. Here, we test whether this reset depends on the time (using different motion speeds) or the distance of the accumulated position errors. We presented three different motion speeds and two different path lengths before the temporal gap, followed by a similar continued motion after the gap. Perceived offsets were similar for these three speeds and were close to the expected illusion size for the shorter path length (deviation from the expected offset −15±18% of path length). However, for the longer path length at the same external speeds, overall perceived offsets were notably less than predicted (deviation −40±12%). There was no main effect of speed or interaction between path length and speed. Our results imply that accumulation of position shifts depends on distance, suggesting a spatial, not temporal, upper limit for the motion-position integration of this stimulus.
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