Does it really exist? Creating the collinear masking effect by illusory contours

Usually objects on a salient line should be easier to be found due to the obvious position these objects located. However, Jingling and Tseng (2013) discovered that when objects are arranged regularly to form a collinear line, a target on this line is harder to be found. This phenomenon is called the collinear masking effect. One possibility that creates the collinear masking effect is a filling-in process of the collinear grouping, making an illusory contour and smearing the visibility of the target. To test the conjecture, we designed several search displays formed by different illusory contours (e.g., Kaniza type and abutting line illusory contours) to examine whether the illusory contour can mask a local target. The results of three experiments showed that targets on an illusory contour actually easier to be found (i.e., faster RT and higher accuracy) than those not on illusory contours. In other words, the collinear masking effect was not due to a perceptual filling-in on illusory contours. This experiment may improve our understanding about the underlying mechanism of the collinear masking effect.