Task and size effects in the Oppel-Kundt and Irradiation illusions

The present study examined the effects of type of task and stimulus size on two well-known illusions of perceived size: the Oppel-Kundt illusion and the Irradiation illusion. In Experiment 1, a forced-choice task, in which 87 male and female college observers chose the "longer" of two side-by-side stimuli, was used to determine the relative perceived size of stimuli of different length that were all white, all black, or white with various numbers of black stripes. In Experiment 2, an absolute judgment task, in which 73 male and female college observers reported their direct estimate of a single target's length, was used for the same determinations. For both kinds of perceptual judgment, the Oppel-Kundt illusion is strongly in evidence (p less than .001), but the magnitude of the illusion does not increase proportionally with the size of the stimuli. The Irradiation illusion is only found with the forced-choice judgments in Experiment 1. Implications for the generalizability of these illusions to nonlaboratory settings are noted.