The Effects of Display and Observer Strategy Variables on Bourdon Assimilation Illusions

When a solid (filled-in) triangle is mirror-reflected about its vertex such that one angle arm and its reflection form a straight edge, this straight edge appears as a chevron bent in the direction of the other sides of the triangles, an effect directionally opposite to the well-known tilt illusion that occurs with outline angles. It has been proposed that these negative solid-angle illusions (assimilation effects) which occur in dual-angle (Bourdon-type) displays result from a failure to discriminate between the test (judged) edges and the bisectors of the solid angles. In dual-angle outline displays, near-zero effects have been attributed to the availability of collinearity cues. These hypotheses were tested in two experiments in which cues to collinearity were reduced by inserting gaps between the angles and in which collinearity information was increased by adding thick ‘necks’ to the displays. The results are consistent with predictions and implicate not only the nature of the angle display, but also the way in which observers perform the matching task in the production of assimilation effects.