Oppel-Kundt Illusion and Visual Field Anisotropy

Psychophysiological experiments are reported in which a combined influence of visual field anisotropy and the Oppel - Kundt illusion on length judgement was tested. The subjects adjusted the test interval of a stimulus to be equal with the reference interval in length. The stimuli—three dots or the Oppel - Kundt figure with ten dots within the filled half—were generated on the monitor. In the Oppel - Kundt figure, the filled half was considered as the reference interval, and the empty half as the test one. In sessions of the experiments, values of errors as functions of the size and orientation of the stimulus were measured. The reference interval varied within 14 – 150 min arc range, and the orientation was fixed in the 0°, 90°, 180°, or 270° position. The orientation of the test interval varied from 0° to 360°, in 7° steps. The experiments with the three-dot stimuli yielded pure characteristics of visual field anisotropy, and the experiments with the Oppel - Kundt figure showed combined appearance of both the anisotropy and the illusion. The data demonstrate an algebraic summation of the two factors in a simultaneous manifestation. It is assumed that estimation of symmetry of the stimulus, in accordance to the bisector of its internal angle, provides discrimination of the length. Calculations have shown a satisfying confirmation of this assumption.