# Please explain this optical illusion **URL:** \ **Category:** Factual Questions\ **Created:** [November 21, 2003, 2:29pm UTC](https://boards.straightdope.com/t/please-explain-this-optical-illusion/214496 "2003-11-21T14:29:43Z")\ **Posts on this page:** 5\ **Page:** 2
**Author:** ![John\_Kentzel-Griffin](https://avatars.discourse-cdn.com/v4/letter/j/ccd318/32.png) [@John\_Kentzel-Griffin](https://boards.straightdope.com/u/John_Kentzel-Griffin)\ **Post date:** [November 22, 2003, 1:18am UTC](https://boards.straightdope.com/t/please-explain-this-optical-illusion/214496/21 "2003-11-22T01:18:51Z")
I don’t have to stare at it for any length of time. The effect is immediate. Also, the effect is just as strong for me with one eye closed. ---
**Author:** ![That\_Insufferable\_Jerk](https://avatars.discourse-cdn.com/v4/letter/t/13edae/32.png) [@That\_Insufferable\_Jerk](https://boards.straightdope.com/u/That_Insufferable_Jerk)\ **Post date:** [November 22, 2003, 3:40am UTC](https://boards.straightdope.com/t/please-explain-this-optical-illusion/214496/22 "2003-11-22T03:40:12Z")
And the answer is… This illusion has to do with the differences in the acuity of you retina. The macula is the most detail sensitive part of the retina. When you look at something, it is this part of the retina that you use. All other areas of the retina have a descending acuity as you reach the outer part of the retina. And this matters why? Because as your focus shifts around the image different parts of the image are being perceived with more or less acuity. When a part of the image goes from high detail (macula) to low detail (not macula) the contrast of the image shifts, (kind of like a dither or a morph) This shifting in the acuity is perceived as movement because the contrast of the image causes it to change shape when viewed at different acuities. And you have a cite for this? No, but I have to experiments that prove it. First, the illusion disappears when eyes are still and focused on the image. (some of you will not be able to see this for the fact that your eyes move in a slight but constant manner) If you want to really notice this put your nose right up to the screen and focus on one pixel. Hold it for a few seconds and the illusion of movement disappears. Second, Take your eyes out of focus when looking at the image. The illusion of movement occupies the entire image when you do this. And second and a half, the illusion only occurs at the edges of your vision. Second and three quarters, if you take out all but one circle the illusion disappears. ---
**Author:** ![Achernar](https://avatars.discourse-cdn.com/v4/letter/a/e274bd/32.png) [@Achernar](https://boards.straightdope.com/u/Achernar)\ **Post date:** [November 22, 2003, 4:05am UTC](https://boards.straightdope.com/t/please-explain-this-optical-illusion/214496/23 "2003-11-22T04:05:10Z")
**That Insufferable Jerk…** , if you’re right, then the colors shouldn’t matter, right? Red-green-orange-tan should work just as well as blue-black-yellow-white? ---
**Author:** ![II\_Gyan\_II](https://avatars.discourse-cdn.com/v4/letter/i/bbe5ce/32.png) [@II\_Gyan\_II](https://boards.straightdope.com/u/II_Gyan_II)\ **Post date:** [November 22, 2003, 4:07am UTC](https://boards.straightdope.com/t/please-explain-this-optical-illusion/214496/24 "2003-11-22T04:07:02Z")
Going on a tangent here, two months back, I came across this optical effect where you stare at a couple of colored rectangles for a couple of minutes, then look at either whitespace or grey rectangles. You’ll see a color imprinting (sort of afterimage). The effect persists longer for extroverts (upto a few hours) than for introverts (so the site said). I spent 2-3 mins. looking for it on Google, but I can’t get the right keywords. Has anyone seen it? ---
**Author:** ![That\_Insufferable\_Jerk](https://avatars.discourse-cdn.com/v4/letter/t/13edae/32.png) [@That\_Insufferable\_Jerk](https://boards.straightdope.com/u/That_Insufferable_Jerk)\ **Post date:** [November 22, 2003, 4:26am UTC](https://boards.straightdope.com/t/please-explain-this-optical-illusion/214496/25 "2003-11-22T04:26:55Z")
The combined contrast of the colors would matter, but i would imagine that black and white images would have the same contrast. [Previous page](https://boards.straightdope.com/t/please-explain-this-optical-illusion/214496.md?page=1)