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Won Mok, Patrick Cavanagh; Illusory displacement of flash location depends on the perceived direction of bistable quartet motion. Journal of Vision 2002;2(7):260. doi: 10.1167/2.7.260.
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Purpose: Motion can influence the perceived position of a nearby static flash (Whitney & Cavanagh, 2000) but these experiments used drifting luminance gratings that strongly activate passive, low-level processes. To investigate whether high-level processes also contribute to the position encoding of stationary objects, we measured the shift in perceived location of a test flash as a function of the perceived direction of a bistable quartet. Methods: Two flashes were briefly presented just outside the square defined by the four corners of the quartet display. The flashes were placed midway along two opposite sides (either left and right sides or top and bottom sides). The tests were presented in every second ISI so that the same direction of motion would be seen at each presentation. Subjects judged whether the direction of the quartet's motion was vertical or horizontal and then, over repeating cycles, adjusted the relative position of the flashes until they appeared aligned. Results: When the perceived motion of the quartet was vertical, the two horizontal flashes (to the left and right of the midpoint of the square defined by the quartet) were adjacent to the motion paths and showed a considerable vertical misalignment in the direction of the motion. However, when the perceived direction of motion was horizontal, along the top and bottom sides of the quartet, there was no illusory displacement for these same flashes. In contrast, the vertically aligned test flashes were displaced when the perceived motion was horizontal but not when it was vertical. Conclusion: Even though there was no change in the physical stimulus, the position shift of the test flashes was determined by the perceived direction of motion — as the motion organization changed, the direction of illusory location shifts followed. This result indicates that high-level motion processes can influence the position encoding of stationary objects.
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