# Why does the rotating ring optical illusion work?

I would like to replicate the type of illusion occurring in this case:

To observe the illusion look at the center dot while adjust the distance your head is from the monitor.

What are the important factors one must preserve from an image like this in order to get the opposed rotation effect associated with this illusion?

Is it a result of the false embossing and pseudo 3D? Are these just simple light/dark interactions? I'm pretty sure the angular offset between the inner and outer rings is important and having some kind of shape directed along the tangent are essential.

• This is not a question of color theory but rather visual psychology and biology. Jun 24, 2017 at 6:34
• @joojaa By color theory I meant more in terms of light and dark interactions, but I dont quite know if there is a tag to describe that Jun 24, 2017 at 6:42
• Color theory is about how to choose colors for artworks. Jun 24, 2017 at 7:51
• Anyway if you flip the image then it is doing the oppsite thing. Jun 24, 2017 at 7:52
• Is there a term light/dark theory then? Jun 24, 2017 at 23:19

You can find the explanation on the Pinna illusion here.

The important points useful to explain this illusion are the following: (i) the micropatterns have oriented low-frequency components, (ii) these engage low-level direction selective mechanisms, which (iii) are subject to the aperture problem. The implicit orientation polarity in the micropatterns (i.e., the low frequency luminance gradients) and not the black and white edges (i.e., the high-frequency components), is the basic attribute underlying this illusion. The notion of implicit orientation suggests that the illusion can be explained in terms of orthogonal biases (Grossberg, Mingolla & Viswanathan, 2001; Gurnsey & Pagé, 2006; Gurnsey et al., 2002; Mather, 2000; Pinna & Brelstaff, 2000; Pinna & Spillmann, 2005), on the basis of which the visual system produces an interpretation of image flow biased towards the strongest velocities perpendicular to the two-dimensional contours in the image. In Figure 10-left, the two bottom micropatterns show a blurred version of the two above. Under these conditions the high frequencies have been removed from the micropatterns. By translating the micropatterns to the right, they will most strongly stimulate neurons selective for the directions indicated by the white arrows. This bias can be considered to occur when the process of optical flow estimation is contaminated by spatiotemporal noise (Fermüler & Malm, 2004; Fermüller, Pless & Aloimonos, 2000; Weiss & Fleet, 2002; Weiss, Simoncelli & Adelson, 2002). More precisely, the interpretation of the motion effect depends on a step where image features such as lines, intersections of lines, black and white edges like those of Figure 1 and local image movement are derived.

Figure 1

Figure 10

These features contain many sources of noise or uncertainty that can cause bias. As a result, the locations of features are perceived erroneously and the appearance of the patterns is altered. Thus, the estimated flow vectors of Figure 1 are biased in the clockwise and in the counterclockwise directions as can be perceived in the outer and inner ring. The role of low-frequency luminance gradients is demonstrated by replacing the micropatterns with Gabor patches (Bayerl & Neumann, 2002; Gurnsey & Pagé, 2006; Gurnsey et al., 2002; Morgan, 2002). In this case, the strength of the illusion persists or is even enhanced (see Figure 10-right). Gurnsey et al. (2002) demonstrated that the strength of the illusion depends on the number of Gabor patches in the display, their wavelengths, and the orientation diﬀerence between adjacent micropatterns in the inner and outer rings. The illusion can be explained by the response of direction-selective neurons at the earliest cortical stage of visual processing, i.e., area V1. These neurons can signal the speed with which a line of its preferred orientation moves through its receptive field. This constraint may be considered as akin to the aperture effect (cf. Nakayama & Silverman, 1988) by which a moving straight line seen through an aperture can be perceived to move only along the direction of its normal. While this seems to explain how individual square elements receive a local illusory motion signal, the illusory rotational motion can be thought to be sensed by the higher cortical area such as MT (medium scale motion analysis, inhibition of opponent directions) and dorsal MST (MSTd – large scale motion analysis, directional decomposition) which collates all the signals provided by the local motion micropatterns. An FMRI study of the illusion showed activation of the motion specific complex hMT+ in addition to the V1/V2 areas to be involved in the perception of the illusion (Budnik et al., 2006).

Credit: Dr. Baingio Pinna, Dipartimento di Scienze dei Linguaggi, Università di Sassari, Italy

There must be recognizable circular form and circularly symmetrical lines that do not point to the center of the symmetry.

No full circle is needed. Scroll the screen so that one pattern is half out of the window and see it yourself.

The effect is stronger, if

• the apparent radius of the circular pattern is bigger, more free space in the middle
• there's another pattern that has different radius, same center and different skewing direction

The centerpoint marker is not necessary, but the eye gets in the right mode more easily if there's one. Try it. Place your mouse to the unmarked center. Low contrast pattern but clear centerpoint also makes it easier co catch the effect.

• Maybe it's subjective, but I get a much more noticeable illusion of rotation from the question's image (with the parallelogram shapes) than from this example (with the dark red lines). I wonder if it's because of the shading on "leading" and "trailing" sides?
– Anko
Jun 24, 2017 at 15:11
• I did a little more digging last night, in the image I posted there are 36 shapes on the outside and 30 on the inside. I suspect that the effect is more pronounced because the eyes try to align then as though there are 36 and 36 respectively Jun 24, 2017 at 17:05