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Isabelle Mareschal, Joshua Solomon, Michael Morgan; Endogenous attention can reduce the tilt illusion, but not crowding. Journal of Vision 2009;9(8):126. doi: 10.1167/9.8.126.
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Outside the fovea, the visual system both accentuates and obscures the differences between adjacent visual stimuli. We adapted Freeman, Sagi & Driver's (2001) dual-task paradigm to determine whether these complementary effects were similarly modifiable with endogenous attention. Left or right of fixation, the slight tilt of an almost horizontal Gabor pattern becomes hard to classify (as clockwise or counter-clockwise of horizontal) when horizontal Gabors appear above and below it. Classification is even more difficult when the tilted target is instead flanked on its left and right. We wondered whether this crowding could be alleviated if observers were required, not only to classify the target's tilt, but also to determine whether the flanks above and below it had the same spatial frequencies. The result was a clear “No.” Acuity for tilt did not increase when attention was thus directed to the vertically aligned flanks. We even found a slight decrease in orientation acuity when attention was similarly directed to the horizontally aligned flanks. When, instead of being horizontal, the flanks are tilted at 20 degrees with respect to the horizontal axis, a physically horizontal target will appear tilted in the other direction. The results show a significant decrease in this tilt illusion when attention was directed to either to the flanks above and below the target or to the flanks on its left and right. Crowding's greater resistance to manipulations of endogenous attention suggest that it reflects a process that is different from and probably earlier than the one responsible for the tilt illusion.
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