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Su-Ling Yeh, Chien-Hui Chiu, Chuan-Heng Hsiao; The Gestaltist's error revisited with sound. Journal of Vision 2008;8(6):175. doi: 10.1167/8.6.175.
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© ARVO (1962-2015); The Authors (2016-present)
Rotation of a fan-like shape at a constant velocity is perceived as “pause-and-go” when the leaves are occluded behind those of a static fan (Petter, 1956). This phenomenon has been attributed to “the Gestaltist's error” (Kanisza, 1979) since only with perceptual grouping of the individual leaves into an object and assuming, like a Gestaltist, that object motion should be continuous even under occlusion will such a phenomenon be called an illusion. Inspired by this intriguing phenomenon (see our demo), we examined why the smooth motion becomes jerky and tested whether adding a sound will affect the motion perception. We manipulated the number of leaves and asked the participants to judge the smoothness of motion in a 2AFC with adaptive staircase procedure. The result showed that reducing the number of leaves enhanced motion smoothness. A collective object account is provided in which (1) the multiple leaves are perceived as multiple objects; (2) attention resource is required for an accurate interpolation of motion speed under occlusion but only for one object at a time; and (3) the moving leaves are treated as a collective object with common motion and thus the motion speeds of each are averaged. We tested this account by adding a sound with different pitch during the occlusion period. Higher threshold speed with sound than without it suggests that sound further disrupts motion smoothness because it attracts attention. Although the possibility of associative learning between jammed sound and slower speed of the fan cannot be excluded, it is not favored because it cannot explain the results of jerky motion with no sound and no effect of pitch on the motion smoothness judgment.
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