Warning: for illusion insiders only
What to do & see
When looking around the above image you should see some illusory rotation. The motion direction directionwith the default settings is from the black stripe towards the neighboring dark gray one. Thus the top left “snake wheel” appears to rotate clockwise, the top center one counter-clockwise. [The checkbox “alternate” changes this alternation.]
If I watch this for an extended time span, the illusion tends to weaken.
As a next step, calibrate the luminance linearity of you screen by ticking the ‘γ’ (gamma) checkbox. During the calibration process a central checkerboard sits on a grey background, whose luminance is controlled by the slider. Try to equate the background luminance with the mean of the checkerboard (blurring the image helps, ideally the checkerboard vanishes).
With calibrated luminance, you can now experiment with the effect of the luminance of the grey stripes straddling the black and white stripes on the strength of the motion illusion. As a first step, pick the pop-up showing 20%·40% and set it to 70%·95%. With that luminance choice, many observers see weak but opposite rotation of the Rotating Snakes.
[One way to convince you of this: with 20%·40%, note that the outer 4 wheels go clockwise, especially when you're not looking at them. Switching to 70%·95% then leads to brief counter-clock rotations.]
Finally, the checkbox “Modulate contrast” allows to observe the effect of onset/offset (with variable speed). In this mode, the illusion obtains even with steady fixation. The perceived illutiondepends depends on the background luminance, adjustable with the top-right slider.
The famous Snake Illusion was first demonstrated by Akiyoshi Kitaoka in 2003.
This page accompanies a (hopefully :) forthcoming publication, based on our 2005 VSS poster.
Kitaoka A, Ashida H (2003) Phenomenal Characteristics of the peripheral drift illusion. VISION: The Journal of the Vision Society of Japan, 15, 261–262
Bach M, Gérard L (2005) Rotating Snakes Illusion – Quantitative analysis reveals islands in luminance space with opposite illusory rotation. JOV 14,10. doi: 10.1167/14.10.267. PDF: f1000research.com/posters/1095954
Atala-Gérard L, Bach M (2017) Rotating Snakes Illusion – Quantitative analysis reveals a region in luminance space with opposite illusory rotation. iPerception 8(1):2041669517691779