Optical illusions: caused by eye or brain?

November 11, 2008 By Lisa Zyga feature
(Left) Isia Leviant’s Enigma. Most viewers say the purple rings appear to fill with rapid illusory motion. (Right) A simplified version that the researchers used in the study. Most viewers report illusory motion lengthwise along the gray stripe. Image credit: Xoana G. Troncoso, et al. ©2008 PNAS.

When viewing the famous optical illusion painting Enigma by Isia Leviant, many people claim to see motion within the colored circles moving against the black and white striped background. Although this optical illusion has been known for a long time, its physiological origins are still unknown.

For the past 200 years, researchers have debated whether the illusion of motion in a static image is caused by mechanisms in the eye, in the brain, or by a combination of both. Because measuring these kinds of physiological responses is difficult, no study has successfully measured direct and tightly timed correlations between a kinetic illusion and a physiological precursor.

But recently, a team of researchers from the Barrow Neurological Institute in Phoenix, Arizona, and the University of Vigo in Vigo, Spain, has found a direct correlation between illusory motion and microsaccades, which are tiny eye movements that involuntarily occur several times per second during visual fixation. Although the team hasn’t determined the neural mechanism behind the correlation, the finding rules out the hypothesis that the origin of the kinetic optical illusion is purely cortical.

“These results revealed a direct link between the eye motions and the perception of illusory motion, and ruled out the hypothesis that the Enigma illusion originates solely in the brain,” Susana Martinez-Conde, Director of the Laboratory of Visual Neuroscience at the Barrow Neurological Institute, told PhysOrg.com. “Our study provides a possible explanation for an entire family of visual illusions central to the fields of visual art and visual science. It would be quite unexpected if Enigma turned out to be the only motion illusion affected by eye movements.”

In tests, the researchers tracked subjects’ eye positions with a video-based eye movement monitor, while asking the subjects to release a button when they perceived fast movement, and press the button when they perceived slow to no movement (no illusion). The results showed that microsaccade rates increased shortly before subjects reported seeing fast movement. Conversely, microsaccade rates decreased shortly before subjects reported seeing slower movement.

These findings show a direct correlation between microsaccades and the perception of illusory motion, but the researchers also wanted to know if microsaccades might actually cause the illusion. To investigate this possibility, the team tested an alternative hypothesis: that, rather than microsaccades causing the illusion, the illusion causes both the microsaccades and subjects’ perceived illusory motion (reported by pressing and releasing the button).

In this control test, subjects were shown real moving images and asked to indicate changes in motion. Results from the eye movement monitor showed that, unlike when viewing the static images, subjects’ microsaccade rates did not increase before transitions to faster movement, nor did microsaccade rates decrease before transitions to slower movement. Further, microsaccade rates actually decreased after a change in motion, whether faster or slower. These findings show that perceptual motion changes do not cause changes in microsaccade rate, answering the first part of the alternative hypothesis.

From this data, the researchers could also determine the time delay between the actual change in motion and the subjects’ reported change in motion, which was about 520 microseconds. Because the measured changes in microsaccade rates occurred about 740 microseconds before this reported change, the researchers concluded that microsaccades may potentially drive the illusion, but not the other way around.

If microsaccades do indeed drive the Enigma illusion, future research will still need to investigate whether the cause is direct or indirect, and what neural mechanisms may be at its root. In previous studies, researchers have proposed a few possibilities based on the idea of “phi movement,” which is the illusion of apparent motion caused by switching off one stimulus and immediately turning on another in close proximity.

For example, microsaccades may cause parts of the image to shift, which could cause the black and white lines to continuously reverse, resulting in phi movement. Another possibility is that microsaccades may displace after-images of the black and white stripes, also causing phi movement. A more complex eye-brain explanation is also possible, and the researchers will continue investigating this intriguing illusion.

By learning more about how optical illusions work, Martinez-Conde and her colleagues hope to understand more about the brain, and how it creates our sensory reality.

“Visual illusions are defined by the dissociation between physical reality and subjective perception of an object or event,” she said. “When we experience a visual illusion, we may see something that is not there, or fail to see something that is there, or even see something different from what is there. Because of this dissociation between perception and reality, visual illusions demonstrate the ways in which the brain can fail to recreate the physical world. By studying these failings, we can learn about the computational methods that the brain uses to construct visual experience. Presently, one of the most important tools used by neuroscientists to understand how the brain creates its sense of reality is the visual illusion.

“The findings from our recent study may moreover help us to understand the neural mechanisms underlying motion perception, both in the normal brain, and in patients with brain lesions that affect the perception of motion,” she said. “Also, they could help design future neural prosthetics for patients with brain damage.”

More information: Troncoso, Xoana G.; Macknik, Stephen L.; Otero-Millan, Jorge; and Martinez-Conde, Susana. “Microsaccades drive illusory motion in the Enigma illusion.” Proceedings of the National Academy of Sciences, October 14, 2008, vol. 105, no. 41, 16033-16038.

Copyright 2008 PhysOrg.com.
All rights reserved. This material may not be published, broadcast, rewritten or redistributed in whole or part without the express written permission of PhysOrg.com.

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Display comments: newest first

1 / 5 (6) Nov 11, 2008
Dude I'll go ask my mind something useful now thanks...
1 / 5 (5) Nov 12, 2008
And you have found nothing!No! You havent even found nothing!
1 / 5 (5) Nov 12, 2008
And to visualize your process it looks like this.

1 / 5 (5) Nov 12, 2008
Hey definitionists and what is time anyway if numbers posses no universal meanings?
1 / 5 (5) Nov 12, 2008
And to visualize your process it looks like this.


And sorry your process is not eco yet!I must have missed the type of the machine.Sorry.
1 / 5 (5) Nov 12, 2008
Hey definitionists and what is time anyway if numbers posses no universal meanings?

And how can you say you were debating if there is no time?And if there is time is your debating included in your time definition?Are all your codes included in time definition?How come some then dont fit together?Is religious god included in your time definition?It should be. But maybe is it your whole time definition that is not important? What is important anyway?Money?I guess money is the most important. If so have you included money in your time definition? You should. But if so the best your existance can be is just a part of time definition. So are you trying to tell me that you are trying to explain a time definition with a part of the same time definition? If so Thank you.
2.3 / 5 (3) Nov 13, 2008
I think you might have looked at the picture for a bit too long there VP.....
1 / 5 (5) Nov 13, 2008
Might be. But i bet everything i have that you dont understand a single word you are saying.
1 / 5 (5) Nov 13, 2008
Or it would be nice if you try to prove me that you do?
1 / 5 (5) Nov 13, 2008
And i have one question also for you. You think equation exist universally?
1 / 5 (5) Nov 13, 2008
I think...

And you care to explain me what thinking process is universally?If not, then im not sure what are you trying to tell me that you think.
1 / 5 (6) Nov 13, 2008
And if your thinking process is the same as mine(as if you would know what it is or as if its already defined) how come we are not thinking the same then?
not rated yet Nov 13, 2008
velvetpink, don't you have anything better to do than spend an entire morning spamming an Internet article? Seriously, get a girl/boyfriend and start living a little.
1 / 5 (5) Nov 13, 2008
And dont you have anything more interesting to say?So you just came here to help me prove my point?If so thank you. And dont worry for my life too much.I was born dead anyway.And may you have a great life my friend and dont care too much cause it may harm your beauty.
1 / 5 (5) Nov 13, 2008
And why you reading anyway if you arent interested in my words.So you really think you are totally controlling your body?Oh no!You thought you are controlling the universe with your mind?Hehe.Not so fast my lovely little humans.There is a long long way to go.And this planet was not made for that way anyway so just keep pushing, your own destruction.Congratultions.
4.9 / 5 (45) Nov 15, 2008
Typical GW nut case.
2 / 5 (1) Nov 17, 2008
The optical illusion above is probably caused by heat in the eyeballs that becomes an interference in an electrical process. To be certain I would have to remember to look at it again when I have walked outside for a long time in freezing weather. Unfortunately, I am not too likely to remember it any time soon... and I don't walk in the cold.

However, some other concentric illusions are likely caused by the movement of the vitreous humour affecting the hyaloid canal. The hyaloid canal is where external light input from the eye is concentrated by the lenses and is turned into electrical processing from the original light input.

Does that check?

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