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Peter Scarfe, Alan Johnston; Motion drag induced by global motion Gabor arrays. Journal of Vision 2009;9(8):656. doi: 10.1167/9.8.656.
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The position of stationary flashed objects can be shifted in the direction of a moving object (Whitney and Cavanagh, 2000). However it is not known whether motion drag is induced by local motion or object motion resulting from the spatial pooling of local motion estimates. Here we investigated this remote positional drag with global motion Gabor arrays (Amano, Edwards, Badcock & Nishida, in press). The arrays consisted of multiple Gabor patches with orientations between plus/minus 90 degrees of the global motion direction. Each 1D Gabor was assigned a drift speed such that local velocities were consistent with the normal components of a single object moving in the global motion direction. Two vertically separated 29 × 9 Gabor arrays (diameter = 0.5°; spacing 0.1°) were placed above and below fixation. On any trail they moved horizontally in opposite directions with the direction chosen at random from trial to trial. During presentation of the arrays two lines were briefly flashed, one above the top array and one below the bottom array. We measured the horizontal separation needed for the two lines to be perceived as horizontally aligned using the method of constant stimuli. We compared the magnitude of the motion-induced positional drag across global motion arrays against the position shift for consistent arrays where all the Gabor's had the same global orientation and speed. We also investigated the effects of reducing the number of Gabor's in each type of array (random deletion from 261 to 87). We found that both types of array induced approximately the same amount of remote positional drag and drag remained approximately constant for both types of array across changes in the number of Gabor's, indicating motion drag is induced after the extraction of the global motion signal.
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