1. The rotated snow tire traffic sign illusion
Marin Dujmović
Pavle Valerjev
Tanja Gulan

2. B45

3. B45

4. B45

5. Simmilar principles? The Delboeuf illusion
It seems there is an underlined process of assimilation
The outer circle is „pulling” the inner circle outward so it looks larger  top right, and bottom left

6. Delboeuf in reverse... The same prinicple
Outer circles are the same size, yet the left one seems larger
Illusion strength varies

7. Experiments 1 & 2 (2014) Two experiments were conducted
Experiment 1 – N = 48 – method of adjustment (using PEBL)
Participants used the arrow keys to adjust the horizontal radius of each target until it looked to them like a perfect circle
The vertical radius was held constant at 200px while the starting horizontal radius varied from px when starting at a lower value, and px when starting from a higher value
control ellipse B45

8. Experiments 1 & 2 (2014) Results of Experiment 1
Participants PSE for the 3 experimental situations
F(2, 94) = 3.84, p<.05
The only significant difference was between the Elipse and Control situations
„Fuzzy” data within subjects  Experiment 2

9. Experiments 1 & 2 (2014)
Experiment 2 – N = 27 - method of constant stimuli
Same 3 experimental situations (elipse, B45, control)
Participants were shown stimulus with a fixed vertical radius of 200px while the horizontal radius varied from 190px to 210px (11 different values x 10 trials at each value x 3 situations = 330 trials)
Participants were instructed to estimate which radius was longer (as seen from a distance of 2 meters) by pressing the appropriate key

10. Experiments 1 & 2 (2014) Results – Experiment 2
F (2, 52) = 6.28, p<.01
PSE for both the elipse and B45 were significantly lower than in the control situation
Participants perceived the horizontal radius of the B45 as larger  as was expected if the illusion is present

11. Fast forward – 2016 – Experiment 3
Revisiting the illusion to test whether it is stable depending on image rotation

12. Experiment 3 N = 30 Method of constant stimuli
As in Experiment 2 – participants were presented with the same 3 experimental situations only rotated
To keep the same retinal image size as in Experiment 2 from a shorter distance, the stimuli were appropriately shrunk
The horizontal radius was held fixed at 50px while the vertical varied from 46-52px – 7 possible values x 20 trials per value x 3 situations = 420 trials
As before participants chose whether the horizontal or vertical radius was longer

13. Experiment 3 - Results F (2, 56) = 5.30, p<.01
PSE of the vertical radius is signifficantly lower in the B45 situation
As expected  When the sign was rotated the vertical dimension seemed distorted to the participants

14. Experiment 3 - results
Calculated Pearson correlation coefficients between PSEs for the 3 experimental situations
Elipse
Control
B45
.29
.32 -
.61**

15. To conclude
The snow tire sign illusion remains stable when the sign is rotated
The results indicate that other factors contribute to the illusion since it is not as stable for a shape such as the elipse used in these experiments
Speculation about the possible additional factors?

16. THANK YOU FOR YOUR ATTENTION