The influence of perceptual grouping on the tilt illusion

Summary

Three experiments investigated the occurrence of the tilt illusion with surrounds whose orientation could be defined by perceptual grouping of their pattern elements. Line segments were arranged so as to produce a perceived organization which, if represented by a continuous grating, would induce a shift in the apparent vertical of a central test field. A larger illusion was obtained when the elements were horizontal (Experiment 1), than when they were themselves oriented so as to produce an effect opposite to that of the perceived global structure (Experiment 2). An effect was also found for a surround pattern with a continuous horizontal physical orientation, but a perceived structure appropriate to induce a tilt illusion. The illusion may therefore be influenced by processes at a higher level than has generally been proposed.