Scintillating Starburst illustrates perceptual illusions

[Most people most of the time don’t think about the fact that they’re experiencing the world only through their senses and perceptual processing systems rather than directly (what some have called first order mediation), but naturally occurring and especially human-created optical illusions (a form of technology defined broadly) can be powerful reminders and represent compelling (tele)presence illusions. The post below from the Pascal’s Pensées, or Pascal’s Thoughts, blog (“Contemplations from the trenches of Neuroscience, Psychology, Metaphysics and Life”) introduces an effective example. See the original version of the post for larger images and a detailed technical report about the illusion. –Matthew]

[Image: A Scintillating Starburst – do you see rays coming from the center?]

Introducing a Visual Illusion – the Scintillating Starburst

Posted on May 27, 2021 by Lascap

This post consists of two parts. The first part is aimed at introducing this new illusion to a general audience. The second part is intended to supplement technical details for specialist readers.

Most people believe that what they see corresponds to reality, a philosophical position called “naive realism“.

This position is challenged by the existence of visual illusions, where perception differs from reality, revealing the subjective nature of our perception.

One such class of visual illusions is known as “illusory contours”. Presented with this illusion, observers perceive edges that are not actually there. The most well-known example of this phenomenon is the “Kanizsa triangle”.

[Image: Kanizsa triangle (1955). Note the illusory white triangle on top seems to cover the rest of the image. Objectively, there is no difference in the brightness of the inside of the white triangle and the background.]

Most observers interpret this scene – assuming one is looking down from above – as a white triangle being on top of three black circles as well as another triangle, not as three pac-men and < ^ > symbols that just randomly happen to align in this configuration just by chance.

This illustrates another interesting principle of perception – there is compelling evidence that the brain favors the most likely (often the simplest) interpretation of a scene. In other words, the brain is routinely “connecting the dots” to fill in information that is not actually there, and has to do so, as not all necessary information is always fully available. These “best guesses” are often accurate, aiding in the survival of the organism.

All of this has been known for many decades.

In the image at the beginning of this post, bright – but fleeting – rays appear to emanate from the center of the image, akin to seeing the sun breaking through the clouds. Thus, we call this effect the “Scintillating Starburst“. However, these shimmering rays are entirely illusory: they’re the result of our brain connecting the dots. The starburst is not physically present.

What distinguishes this illusion from known effects is that in the Kanizsa triangle, the inducers (the pac-men) are luminance defined, whereas in our Scintillating Starburst, they are themselves the result of subtle features of our visual system.

Without getting too technical here (for details, see our paper), the Scintillating Starburst illusion can be explained as such: The black concentric “wreaths” (technically pairs of scaled star polygons) are all uniformly colored, but the part of our visual system that processes information from the periphery sees the intersection points as brighter than the rest of the wreath. As these “beacons” of brightness are aligned in linear fashion (along a ray projecting from the center), we believe the brain is connecting the dots accordingly, which is why most people see these illusory rays. What makes them shimmer or scintillate is the fact that another part of the visual system (that which processes information at the center of gaze) does not see the intersection points as brighter, but rather as they actually are. Thus, these rays will be fleeting due to the dynamic interplay between these two systems.

The phenomenology of this effect can be quite striking, and further enhanced by optimizing all stimulus dimensions (see paper) and by rotational motion.

[Image: A rotating Scintillating Starburst]

Thus, the Scintillating Starburst is perhaps best understood as a “compound illusion” combining – and revealing – several features of our visual system, much like the “Lilac chaser”.

People readily interpret their environment in light of incomplete or unreliable information. For instance, when looking at stars, some observers are prone to see constellations. We believe the tendency of some people to “connect the dots” is related to their propensity to see non-existing ray patterns, for the same reason.

Note: This illusion was a finalist in the 2020 “best illusion of the year” contest.

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