Con-fusing contours & pieces of glass

We present a new illusory display in which illusory contours are misaligned with physical contours. In these displays, illusory Kanizsa squares, induced by the so-called pacmen, are positioned on top of a background grid of bars. The misalignment of the illusory contours with respect to physical contours of the grid of bars induces an overall 'restless' appearance and evokes the impression that parts of the grid within the illusory square are shifted. To test this impression, we created stimuli in which illusory squares were superimposed on a grid at different positions, where the grid could consist of either straight bars or indented bars. After briefly flashing these stimuli, observers reported indentations of the background grid for those cases in which physical and illusory contours were misaligned. In a control condition, the pacmen were replaced by crosses (not inducing an illusory square) at the same positions; as expected much less illusory shifts were reported in this condition. In a second experiment, we further tested the direction of the perceived shifts, revealing similar trends as in the first experiment and a consistent result with respect to the reported direction of the shifts. We explore and discuss possible underlying mechanisms with regard to our illusory display.