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Gerrit W. Maus, Jason Fischer, David Whitney; Crowded by drifting Gabors: Is crowding based on physical or perceived stimulus position?. Journal of Vision 2010;10(7):1349. doi: 10.1167/10.7.1349.
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© ARVO (1962-2015); The Authors (2016-present)
In crowding a stimulus in the periphery (the target) becomes hard to recognize when other stimuli (crowders) are presented nearby. Crowding depends on the distance of the crowders to the target (Bouma, 1970). If the position of the crowders is misperceived, which position matters for the degree of crowding, the physical or the perceived position? In the present experiment the crowders were drifting Gabor gratings whose position appeared as shifted in the direction of motion (DeValois & DeValois, 1991). Six gratings, either drifting inward or outward, surrounded a peripheral target grating at systematically varied distances. Observers judged the orientation of the target grating, that could be tilted 2 deg clockwise or counter-clockwise. Observers performed worse, i.e., they experienced more crowding, when gratings were drifting towards the target and thus perceived in a position closer to the target. The difference in performance between inward and outward drifting crowders corresponds to the difference in perceived positions, as determined in a separate psychophysical assessment of the size of the mislocalization illusion. The extent of crowding was fully determined by the perceived position, which in the present case is only available after the integration of motion information. Arguably, feedback from motion-sensitive extra-striate areas is involved in the motion mislocalization illusion. If so, the results demonstrate that crowding is not based solely on spatial interactions in the feed-forward stream. Furthermore, the present results indicate that the influence of motion on position perception occurs early, before low-level features such as orientation can be perceptually judged.
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