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Keith A. May, Li Zhaoping; Both cognitive factors and local inhibition mediate the effect of a surrounding frame in visual search for oriented bars. Journal of Vision 2005;5(8):959. doi: 10.1167/5.8.959.
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It is easier to search for tilted line elements amongst vertical distractors than vice-versa (Treisman & Gormican, 1988, Psychological Review, 95, 15–48). When a vertical or tilted square frame surrounds the elements, there is an advantage for targets tilted relative to the frame. Treisman suggested two explanations: (1) the frame defines the orientation against which tilt is defined, and targets parallel to the frame lack a “tilt” feature, making them harder to find; (2) targets tilted relative to the frame have a unique orientation, making them more salient than targets parallel to the frame, which receive competition from it. Li (2002, Trends in Cognitive Sciences, 6, 9–16) proposed a saliency mechanism that explains these results using iso-orientation inhibition between nearby V1 cells: cells responding to an element parallel to the frame receive more inhibition than those responding to an element with a unique orientation. We ran several experiments to test this model. In each stimulus, either the target or distractors were parallel to the left and right sides of the frame, and no element was parallel to the frame's top and bottom. In experiment 1 the left and right sides of the frame were constructed from elements oriented parallel to the frame's top and bottom; in experiment 2, the left and right sides were removed altogether. Both modifications caused the target to be uniquely oriented whether or not it was tilted relative to the frame and, in both cases, the frame effect was still present (but reduced in experiment 2). These results are not explained by the V1 model, and suggest a role for more cognitive factors. However, other results supported the V1 model, which predicts that inhibition decreases with increasing distance between receptive fields. We found that enlarging the frame, so that it was further from the elements, reduced its effect. In addition, a single line through the stimulus has the same effect as a frame only when the target is close to it.
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