Conference “Biomedical Engineering“ 105 Displacement of the edges of the filled intervals in the Oppel-Kundt illusion T. Surkys 1 , A. Bulatov Institute of Biological Systems and Genetics Research, Lithuanian University of Health Sciences, Lithuania 1 E-mail: surkys@vision.kmu.lt Introduction. In the Oppel-Kundt illusion a filled visual area appears to be larger than an empty area of equal size. Typical Oppel-Kundt illusory pattern comprises two contiguous spatial intervals: one filled with a regular sequence of dividers (stripes or spots), and the other empty one. It is known that the illusion is most prominent when the filled interval is divided into 5 - 10 parts [1, 2]. In our previous work [2] we have showed that the illusion exists also for figures comprising three spatial intervals: either when a filled interval is flanked by two empty ones or when an empty interval is situated between two filled. It was demonstrated that the illusion was greatest when the empty interval was placed between two filled ones, and that the illusion magnitude is defined mainly by changes in length of filled parts of stimuli. Fig. 1. Example of stimuli used in experiments In the present pilot work we have raised a question whether the perceived distortions of length may be caused by local displacements of the edges of the filled intervals of the Oppel-Kundt figure rather than by their homogeneous expansion. It was also interesting to investigate how the illusion depends on the number of dividers and the length of the filled intervals of stimuli. In the stimuli used, the length of the empty interval situated between two filled parts of the Oppel-Kundt figure could be compared with that of separate test interval placed below the figure (Fig. 1). Such organization of stimuli let us to change spatial parameters of flanking filled intervals and maintain the same length of the central empty part. Methods. Visual stimuli were generated by the Cambridge Research Systems VSG 2/3 and presented on the monitor EIZO T562. The monitor was