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Yury Petrov, Ariella Popple, Suzanne McKee; Crowding is directed to the fovea and preserves only feature contrast. Journal of Vision 2007;7(9):594. doi: 10.1167/7.9.594.
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© ARVO (1962-2015); The Authors (2016-present)
The abundant literature on crowding offers fairly simple explanations for the phenomenon, such as position uncertainty or feature pooling, but convincing evidence to support these explanations is lacking. In part, this is because the stimuli traditionally used for crowding studies are characters or other complex shapes, which makes it hard to determine exactly what kind of information is lost. In our first experiment a 45% contrast Gabor target presented at 9 deg eccentricity was flanked by a plaid made of two replicas of the target (one was rotated by 90 deg). The scale of the whole stimulus was varied in an adaptive fashion. Observers had to identify the target's slant (45 deg left or right). In agreement with early crowding studies (e.g. Bouma, 1973) we observed a strong crowding effect, when the mask was positioned outward with respect to the target, but not when positioned inward, above, or below it. This demonstrates that crowding has a pronounced foveal directionality, which rules out explanations based on simple pooling or surround suppression (which does not show such directionality). In our second experiment, we asked observers to identify simultaneously the slants (left or right) of three horizontally aligned Gabor targets. The targets were presented at 6 deg in the periphery, and their size and separation was chosen to incur strong crowding. In agreement with the first experiment we observed that the outmost target was crowded much less than the other targets. The specific pattern of confusion shown by all the observers indicates that the only reliable information available to them was orientation contrast, i.e. the number (and, to a lesser degree, the location) of sites where slant changed. Thus, crowding appears to spare only the most salient peripheral information, which supports the hypothesis that crowding is caused by limitations of attentional resolution.
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