Visual field differences in the magnitude of the Oppel-Kundt illusion vary with processing time.

Three experiments were performed to investigate hemispheric differences in susceptibility to the Oppel-Kundt illusion presented tachistoscopically to the two visual hemifields. Experiment 1 used a successive comparison mode, in which 16 undergraduate students indicated whether the second of two successive extents looked shorter or longer than the first. Experiment 2 (20 under-graduates) and Experiment 3 (1 commissurotomy patient) required judgments of two extents presented simultaneously. The first experiment found no significant visual field differences, although females were more susceptible than males. In the second experiment, the illusion magnitude was greater in the left visual field, and in the third experiment, it was greater in the right visual field. Post hoc analyses resolve the conflicting results and show that in all three experiments, the susceptibility of the right hemisphere declined more than that of the left hemisphere during illusion processing. An interpretation is offered in terms of two parallel processes, one that is fast, uses feature extraction, and is performed more effectively in the left hemisphere, and one that is slow, uses visuospatial analysis to compute distances between parts of the illusion figure, and is performed more effectively in the right hemisphere.