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Li Zhou, Li Zhaoping; The salience of absence: when a hole is more than the sum of its parts. Journal of Vision 2010;10(7):1281. doi: 10.1167/10.7.1281.
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An item can be conspicuous against a uniform background either by possessing a feature other items lack, or, typically to a lesser degree, by lacking a feature the others share. It has always been assumed that the conspicuity of feature absence arises from the saliency at its location. However, if the salience of a location is determined by the largest neural response from primary visual cortex (V1) that it inspires, as suggested by a recent theory, then the only way an absence or a hole could become conspicuous is if the saliency of its surrounding stimuli attracts attention to its vicinity (Li, 2002); it would lead to no V1 activity by itself. Specifically, the absence of input at the hole reduces suppression of the V1 responses to the stimuli surrounding it, in a way that depends on spatial- and feature-specific suppression between nearby V1 neurons, making the surrounding stimuli more salient. If this enhanced saliency of the surround determines the conspicuity of the hole, then altering the visual input strength to those surrounding stimuli should alter the reaction time (RT) for finding the hole in a visual search task, in a way that is predictable from V1 interactions. We test this prediction by measuring observers' RTs for finding a target among distractor crosses when the target consists of just one of the two bars of a cross. When the target bar has sufficiently low contrast, the RT does not increase when its contrast is reduced further, indicating that the V1 response it evokes is immaterial to its conspicuity. Meanwhile, changing the contrast of various bars in the surrounding crosses alters the RTs according to the feature and spatial specificities of V1 interactions. The saliency of a hole may only be a subsequent impression inferred from our perceptual experience.
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