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Mark Chappell, David Hardwick, Trevor J Hine; Combining the Poggendorff and flash-lag illusions. Journal of Vision 2003;3(9):212. doi: https://doi.org/10.1167/3.9.212.
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The static Poggendorff and dynamic flash-lag illusions would arguably be principally processed by the ventral and dorsal streams, respectively, so we sought to investigate the interaction of these streams by combining the illusions. Two horizontal lines were vertically offset by 3 deg. A diagonal line at 45 deg. to the horizontal, whose upper end touched the lower horizontal line, traversed the display at 12 deg./sec. When the moving line was near the centre of the display, another diagonal line, parallel to the first, was flashed, with its lower end touching the upper horizontal line. The horizontal offset between these diagonal lines was varied. The participant's task was to say if the flashed line was to the left or the right of an imaginary line extending from the moving diagonal line and logistic regression was then used to find the point of subjective alignment for the two diagonal lines. We hypothesized that when the diagonal line was leaning to the right and moving to the right the two illusions would cancel each other, whereas when this right-leaning line was moving to the left the illusions should add, so that the displayed offset between the two lines for subjective alignment would be smaller in the former case than in the latter. If the illusions were literally additive, it should be possible to solve simultaneous equations to estimate the size of each of them, and these estimates should compare with estimates for a static Poggendorff illusion and regular flash-lag illusion. Pilot experiments yielded 5 out of 8 participants whose data yielded a significant difference between the conditions described above, but investigations continue to eliminate a range of confounding variables. A surprising asymmetry between leaning-right and leaning-left trials was found. We believe that combining illusions in this way will yield new insights into the mechanisms of visual perception.
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