I saw the image below on facebook. At first, I thought it's a GIF animation since it doesn't appear static but it turned out to be a static JPEG image and the apparent motion is caused by the brain. So why does my brain trick me into believing that there's something moving? Does the brain have a purpose to do this optical illusion?


  • $\begingroup$ Fish responded similarly to real and illusory motion. Complex global illusory motion is not restricted to humans and can be found even in species that do not have a cortex. www.nature.com/srep/2014/140923/srep06443/full/srep06443.html $\endgroup$
    – user7796
    Mar 1 '15 at 15:45

This is a type of illusory motion (or motion illusion) called the Enigma Illusion.

enter image description here

The cause of motion illusion in general is not well understood, but research suggests that there may be slightly different reasons for the different types of motion illusions.

A common theory is that particularly high-contrast colours are perceived separately in the retina (perhaps turning photoreceptor cells on takes a slightly different amount of time than turning them off), and as a result, the signals from the retina arrive at the visual cortex at slightly different times, causing the illusion of motion. Comparison of brain activity using fMRI between actually moving and apparently moving images shows similar activity in the visual cortex, supporting the assertion that the illusion is caused at the retina rather than at the visual cortex.

In the particular case of the Enigma Illusion:

When the eye movements, called microsaccades, were suppressed, test subjects reported that the Enigma illusion — an illustration that seems to flicker and turn — remained stationary.

That is, small, rapid, unconscious eye movements are also involved in creating the illusion.

Here is a nice collection of different illusory motion images: http://brainden.com/eye-illusions.htm

The reason for optical illusions such as this one is a side-effect of the way the brain processes typical visual information efficiently, at the expense of handling unusual situations incorrectly.

  • 1
    $\begingroup$ The article linked to your question supposedly supporting evidence for a retinal origin is actually build around a model in cortical V1. Furthermore, what different cells should be perceiving high-contrast colors? Of course sharp edges can induce strong ON/OFF responses but you may want to address which retinal cells are involved. Lastly, why should different classes of cones have different response times? The only difference between them is a slight modification in the photopigment. I have never heard of R/G/B cones having different temporal characteristics. $\endgroup$
    – AliceD
    Dec 14 '14 at 12:20
  • $\begingroup$ I like your substitute illusion by the way. It definitely is related, and also very strong! I've never seen it before. I'll definitely skim your recommended site for more of these :) $\endgroup$
    – AliceD
    Dec 14 '14 at 12:22
  • $\begingroup$ Good feedback @Chris, thanks. I've updated the answer for clarity; hopefully it makes more sense now. $\endgroup$
    – Arnon Weinberg
    Dec 14 '14 at 18:21
  • $\begingroup$ It does! Thank you. $\endgroup$
    – AliceD
    Dec 15 '14 at 3:25

Mark Changizi proposed another level of understanding this illusion based on the "Perceiving the Present" hypothesis. Here is the explanation:

Optical illusions are actually a result of our brains trying to predict the future. When light hits our retina, it takes about one-tenth of a second for our brain to translate that signal into perception. This neural delay makes our brains generate images of what it thinks the world will look like in one-tenth of a second. It's not always right.

“Your brain is slow, so you need to basically create perceptions that correct for that delay,” said Changizi, director of human cognition at 2AI Labs.

Creating an image of the very near future probably kept early humans alive because it kept them from bumping into dangerous objects or being attacked by a fast-moving predator.

To learn more about the "Perceiving the Present" hypothesis


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