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Justin Ales, Gideon Caplovitz, Anthony Norcia; Neural correlates of perceptual grouping in the occluded diamond illusion. Journal of Vision 2009;9(8):901. doi: 10.1167/9.8.901.
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© ARVO (1962-2015); The Authors (2016-present)
Scene segmentation in cluttered environments relies on the use of global features to interpret ambiguous local features. Changes in the spatial context of moving objects can dramatically change the interpretation of identical motion inputs. A clear demonstration that the spatial context can control how local motion information is interpreted is the Occluded Diamond illusion (Lorenceau and Shiffrar, 1992) in which the perceived motion of local elements is modulated by the presence of an occluding surface. We created a version of this illusion in which one set of bars oscillates periodically at 2 Hz up and down and the other set oscillates left and right at the same frequency. When the two groups are presented on a uniform background, they each appear to move independently. However, when visible patches simulating an occluding surface are placed between them, the bars group together and move coherently as a unit.
We studied the modulatory effect of occluding surfaces on cortical activity by recording high-density EEG in 4 subjects. While the occluder itself evokes no time-locked activity, it strongly modulates the perceptual interpretation and the neural response. The dominant effect on the EEG is an increase in the amplitude of the 4th harmonic response when the occluder is present and the oscillating bars are seen as a single coherent object. This effect is consistent with a two stage model in which the first stage comprises motion energy units that are separately responsive to the orthogonal inputs and a second non-linear grouping stage that pools across orientations when the occluder is present. Rectified motion signals from the first stage that are combined via a second-order non-linearity at the second stage of the model will result in non-linear interaction terms that are fourth order with respect to the input.
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