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Rick H. Cai, Patrick Cavanagh; Motion interpolation of a unique feature into stimulus gaps and blind spots. Journal of Vision 2002;2(7):30. doi: 10.1167/2.7.30.
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Purpose: When a moving bar gradually shrinking in size also changes to a different color for one frame, the odd color is seen as belonging to a smaller bar further along the motion path (Cai & Schlag, VSS 2001). One explanation of this illusory shift in position and size is that the abrupt color change may just be delayed and assigned to a later occurring bar. Here we present experiments to test this possibility. Method: In the first experiment, a gap was inserted into the trajectory of the moving and shrinking bar. A sudden color change occurred immediately before the gap. In the second experiment, the odd bar was longer than the other bars which were all of the same length. The bar's path of motion started near fixation, passed through the blind spot and continued beyond it. The one frame with the longer bar was positioned to fall just outside the blind spot in monocular viewing. In both experiments, observers judged the apparent location of the odd bar. Result: In the first experiment, subjects perceived the odd-colored bar to be in the middle of the gap, with its size being the size of the bar that would have appeared at that location had there been no gap. In the second experiment, the long bar was perceived to lie within the blind spot. Conclusion: Since there is no incoming sense data in the gap in the first condition, the odd-colored feature could not have been assigned to any stimuli actually occurring there. The changing values of size and position must have been interpolated into the gap, with the assignment of color being delayed relative to the construction of the size and location of the bar. In the second condition, this path interpolation continued even into the blind spot, creating a unique visual percept wholly within the blind spot. We suggest that the brain represents continuous and abrupt changes in fundamentally different ways, with the assignment of abrupt changes being delayed.
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