“Rotating Snakes” with luminance control

from Michael’s   Visual Phenomena & Optical Illusions

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[Warning: for illusion aficionados only]

What to do

When looking around the above static image you might see illusory rotation. Use high display brightness for stronger effect. The top left and right “snake wheels” should appear to rotate clockwise, the top center one counter-clockwise, especially if you’re not fixating them. If I watch this for an extended time span, the illusion tends to weaken. The arrangement is a simplified version of Kitaoka’s original that allows me to control colour and luminance of the elements, which are now small wedges.

Element sequence and seeming motion direction

As detailed on the previous page, this motion illusion requires an asymmetric sequence of the individual elements (wedges), e.g. …black-yellow-white-blue-black…. If this sequence is reversed using the checkbox “Alternate”, the seeming motion changes its direction. The perceived motion direction is from the black stripe towards the neighbouring dark grey one (or from the white to the neighbouring light grey).

Illusion versus non-illusion

When the asymmetric sequence is replaced by a symmetric one, namely …black-blue-yellow-white-yellow-blue-black…, the illusion vanishes. This can be tested with the checkbox “Illusion”. The asymmetric sequence requires 6 steps, the symmetric one 4 steps.
I find this effect very strong: while the global aspects of the two variants (with and w/o Illusion) are very similar, the loss of motion is striking in the second case.
Correction: in this mode, there still is some illusion, but a different one, as first noted by George Mather: A slight shimmering, somewhat alike to Enigma or Op-Art; likely related to microsaccades.

With onset/offset, no eye movements necessary

The checkbox “Modulate contr.” allows to observe the effect of temporal contrast modulation with variable speed. Tick the checkbox and notice that the picture alternates between full and very low contrast. In this mode, you can fixate anywhere, e.g. the center of a disk, and notice that after on- or offset there is a brief seeming rotation of opposite direction. This observation plays a role in our new simple computational model (2020, see below).

Colour necessary?

The pop-up menu is initially set to “Orig. colour”, with values taken from Kitaoka’s original picture. By selecting the 20%·40% entry, the colours are replaced by grey levels of ≈equal luminance. The illusion still occurs! This makes it easier to study it quantitatively, and the modifications described above are all applicable.

Gamma calibration

For more quantitative assessment, the luminance linearity of your screen should be calibrated. Ticking the ‘γ’ (gamma) checkbox, and a central checkerboard appears 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). If they are ≈equal, you have found the correct gamma value and the percent grey levels will be correct.

Quantitative influence of luminance

With calibrated luminance, you can now experiment with the effect of local luminance (of the grey wedges straddling the black and white ones) 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. Or use the Modulate contr. setting.]

Comments

The famous Rotating Snakes Illusion was first demonstrated by Akiyoshi Kitaoka in 2003. See previous page for more.

The present page accompanies our 2017-publication “Rotating Snakes Illusion – Quantitative analysis reveals islands in luminance space with opposite illusory rotation”.

We just (2020) calculated a simple model, due ot appear in i-Perception ((preprint here)) showing that this illusion is a trivial consequence when you couple motion detectors into arrays with a little non-linearity.

Sources

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

Bach M, Atala-Gérard L (to be published in i-Percetion), preprint: Bach M, Atala-Gérard L 2020 The Rotating Snake Illusion is a straightforward consequence of non-linearity in arrays of standard motion detectors. BioRXiv PDF

Kitaoka A, Ashida H (2003) Phenomenal Characteristics of the peripheral drift illusion. VISION: The Journal of the Vision Society of Japan, 15, 261–262