ABSTRACT
Do illusory effects cumulate? It has often been suggested that several geometric illusions can be combined into a single display (Gillam, 1991; Schiffman, 1990). Here we ask what actually happens when two illusions are combined. There are several possibilities. The illusions can simply reaffirm each other. In this case, if one illusion made a line look 10% bigger than another, adding a second illusion that also made the line look 10% bigger would make no change. The second illusion would simply reaffirm the results of the first. Alternatively, the effects of two illusions that work in the same direction could accumulate. In this case combining the two illusions would make the line look more than 10% bigger. Consider now combining two “antagonistic” illusions, one making the line look bigger, and the other making the line look smaller. If illusion effects are cumulative, antagonistic illusions could cancel each other out.
