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L. Spillmann, B. Pinna, W. H. Ehrenstein; Dependence of binnenkontrast on the luminance profile of surrounds. Journal of Vision 2001;1(3):444. doi: 10.1167/1.3.444.
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Purpose: To probe the phenomenology of perceptive fields in human vision. Center-surround organization of receptive fields predicts that the optimal stimulus for an on-center neuron is a small white disk surrounded by a black annulus. Due to contrast, the central disk should appear brighter than a uniform reference outside of the stimulus. Method: We tested this prediction using a white disk of 0.7 deg diameter surrounded by a contiguous annulus of 0.7 deg inner diameter and 3.2 deg outer diameter. The surround had one of three luminance profiles: (i) a square decrement, (ii) a negative ramp (becoming darker towards the outside), or (iii) a negative sawtooth (becoming first darker then brighter again). Stimuli were arranged in three arrays of 4 × 5 stimuli each. A total of 10 subjects judged the quality and strength of the brightness perceived in the central disks. Results: Paradoxically, all observers reported seeing bright disks with dark illusory spots (Binnenkontrast) in the middle similar to the diffuse darkenings in a Hermann grid. The spots were less pronounced in foveal than in peripheral vision; they looked slightly smaller for condition (iii) than for condition (ii); and they were surrounded by a bright halo of 0.1 – 0.15 deg. With increasing observation distance (i.e., decreasing disk size), the illusory spots became weaker and disappeared at a disk diameter of 5 arcmin. Conclusion: The occurrence of the dark illusory spots is best explained by a gradient of lateral inhibition, being highest in the center of the disks and lowest at the edge. When the disks are sufficiently small, there is only border contrast and thus no darkening. The visual angle at which this happens agrees with the known diameter of foveal perceptive field centers (Spillmann 1971).
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