The slalom illusion in the context of illusory lines Gheorghes, T. N., Reidy, J., & Soranzo, A. Department of Psychology, Sociology & Politics, Sheffield Hallam University t.gheorghes@shu.ac.uk Introduction • In the slalom illusion, the straight trajectory of a dot crossing a pattern of tilted lines is perceived as being sinusoidal (Cesaro & Agostini, 1988). The perceived trajectory of the moving dot bends to enter the line perpendicularly, generating a local distortion at each line trajectory intersection (fig. 1). o The explanation offered by Cesaro & Agostini (1998) was that the illusion results from the integration of local distortions occurring at the intersections with the tilted lines. This explanation is in line with those proposed by Nihei (1973), after investigating the kinetic version of the Poggendorff illusion and by Swanston (1975) after investigating the kinetic versions the Zollner illusion. • Subjective, or illusory, contours are contours perceived in the absence of any difference in luminescence, colour, or texture. They are the product of certain combinations of incomplete figures (Kanizsa, 1976). o It has been demonstrated that when the solid parallel lines in the Poggendorff illusion are replaced with Kanizsa-type illusory ones, the Poggendorff effect persists, despite a decrease in its magnitude (Tibber et al., 2008). • The present study investigates weather a kinetic illusion of direction, namely the slalom illusion, persists when the inducing lines are based on subjective contours. In doing so, it aims to test the hypothesis that a distortion at retinal level is a necessary condition for the illusion to occur. Methods • Participants - 67 participants naive as to the purpose of the experiment. All participants had normal, or corrected to normal, vision. • Design - Within subjects design with 4 conditions: (1)black inducing lines, (2)subjective contours, (3) grey inducing lines (4)control • Apparatus & stimuli The stimuli (Fig. 2) are based on the original slalom illusion and have the same characteristics throughout the three experimental conditions except for the colour of the inducing lines. In the control condition, the lines have been placed vertically. The stimuli were presented on a Samsung tablet with a 10.1" screen. • Procedure Participants were asked to draw with their finger on the tablet the perceived trajectory of the dot and the dependent variable (magnitude of the illusion) was calculated as the difference between the highest and the lowest peak in the respective drawing. References Cesaro, A. L., & Agostini, T. (1998). The trajectory of a dot crossing a pattern of tilted lines is misperceived . Perception & Psychophysics, 60, 518–523. Tibber, M. S., Melmoth, D. R., & Morgan, M. J. (2008). Biases and sensitivities in the Poggendorff effect when driven by subjective contours. Invest. Ophthalmol. Vis. Sci., 49, 474–478. Kanizsa, G. (1976). Subjective contours. Scientific American, 234, 48–52. Nihei, Y. (1973). A preliminary study on the geometrical illusion of motion path: The kinetic illusion. Tohoku Psychologica Folia, 32, 108– 114. Swanston, M. T. (1984). Displacement of the path of perceived movement by intersection with static contours. Perception & Psychophysics, 36, 324–328. Results & Discussion • Repeated measures ANOVA showed significant differences across the conditions [F(3, 66)=19.63; p<0.001; ηp2=0.23]. A post hoc comparison using Bonferroni correction revealed a significant difference between both the Black Inducing Lines vs Illusory Inducing Lines (p<0.001) and the Grey Inducing Lines vs Illusory Inducing Lines (p<0.001). However, there was no significant difference between Black Inducing Lines vs Grey Inducing Lines (p = 0.22). • As can be seen in the graph, no illusion was observed in the subjective contours condition. It seems that a distortion at the retinal level is a necessary condition for the kinetic illusion to occur. Fig. 1 - The original slalom illusion and its components: a dot crossing on a straight trajectory and at constant speed 6½ modules of tilted lines. The magnitude of the illusion is known to depend on: (i) moving dot speed (ii) angle of intersection, and (iii) distance between the tilted lines. Fig. 2 - Examples of the stimuli used in the four conditions: (1) Black inducing lines, (2) Illusory inducing lines, (3) Grey inducing lines and (4) control condition -2 0 2 4 6 8 10 Illusion magnitude (exp-control) Fig. 3 – Results of the experiment. Bars indicate standard error. ense. F1000 Posters: Use Permi Commons License. F1000 Posters: Use Permitted under Cre der Creative Commons License. F1000 Posters: Use Permitted under Creative Commo Permitted under Creative Commons License. F1000 Posters: Use Permitted under Creative Commons Licen Posters: Use Permitted under Creative Commons License. F1000 Posters: Use Permitted under Creative Co osters: Use Permitted under Creative Commons License. F1000 Posters: Use Permitted unde ermitted under Creative Commons License. F1000 Posters: Use P er Creative Commons License. F1000 P