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Updated: Feb 27, 2026

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Published on: November 30, 2018
An effect of continuous contextual filling in the filled-space illusion
Aleksandr Bulatov1, Natalija Bulatova2, Tadas Surkys3
1Laboratory of Visual Neurophysiology, Lithuanian University of Health Sciences, Kaunas, Lithuania, and Institute of Biological Systems and Genetics Research, Lithuanian University of Health Sciences, Kaunas, Lithuania; bulatov@vision.lsmuni.lt.
Abstract:
In the filled-space (or Oppel-Kundt) illusion, the filled part of the stimulus for most observers appears longer in comparison with the empty one. In the first two experimental series of the present study, we investigated the illusory effect as a function of continuous filling (by a shaft-line segment) of the reference spatial interval of the three-dot stimulus. It was demonstrated that for the fixed length of the reference interval, the magnitude of the illusion increases non-linearly with the shaft length. For the fixed length of the shaft, the illusion magnitude gradually decreases with the lengthening of the reference interval. In the third series, psychophysical examination of the conventional Oppel-Kundt stimulus with different number of equally spaced elements (dots) subdividing its filled part was performed. Based on the analysis of the functional dependencies established, we have proposed a simple computational model that was successfully applied to fit the experimental data obtained in the present study.
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