Theories of human vision have generally assumed that the features underlying visual search and texture segmentation correspond to simple measurements made at the first stages of visual processing. In this paper, we describe a series of visual search experiments that refute this assumption. Using several variants of the Mueller-Lyer figure, we show that an illusion of length exists in preattentive vision -- search is easy when items contain line segments of equal length, but becomes difficult when these segments are adjusted to have the same apparent length. This illusion cannot be reduced by selective inhibition of features, such as that used to facilitate the rapid detection of feature conjunctions. For example, subjects are unable to ignore the wings when making judgements of the test line, even when it is advantageous to do so. This rules out explanations based on interactions among the features themselves. We also show that spatial filtering cannot account for this illusion, since these effects are indifferent to the sign of contrast of the line segments and can occur for textured lines having the same first-order statistics. The illusion, however, can be explained by a model in which line length is determined via grouping operations acting at a level prior to the formation of preattentive features.
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