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I found this image on Google+

enter image description here

If you shake your head you can see a portrait of a person. Can anyone explain how the image is constructed in the brain?

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    $\begingroup$ Does anyone know who made the first version of this illusion and when? also could someone give me a simple explanation of how it works, i'm not too good at all these complicated words! $\endgroup$
    – user3728
    Commented Jun 4, 2013 at 15:41

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If you zoom in on the image, you can see that it is not just composed of black vertical lines, but also has pixels with different gray tones in the white areas. When you move your head sideways, you perceive the gray tones more.

If you were to remove the black lines, you could see the face clearly. Initially I thought that by blurring the gray shapes when your head moved, they became more visible as they seemed larger. On reflection though I think that actually what's happening is that the high contrast between the black lines and the mostly white background causes your perception to adjust so it doesn't easily see the mid tones. This is because we have a low dynamic range in our vision (relative to absolute brightness, but compared to camera CCDs we have a high dynamic range) - we have to adjust the light sensitivity to compensate for the overall brightness of the image. This is called brightness adaptation. There's a good free textbook for further reading about this at Utah U's Webvision.

When you move your head, the black and white lines blur together which makes the overall brightness appear to be the average brightness of the black and white. So against that background your light sensitivity increases and the areas where the pixel tone is different from the average - the gray pixels of the face - start to stand out.

By reducing the brightness you can see the faint image in the background much more clearly...

enter image description here

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    $\begingroup$ Nice answer. Thanks a lot. Just one thing, you said the high contrast between the black lines and the mostly white background causes your perception to adjust so it doesn't easily see the mid tones Does this phenomenon have a specific name? Can you provide references? $\endgroup$
    – Green Noob
    Commented Jul 5, 2012 at 15:55
  • $\begingroup$ Added terminology and a reference. Sorry I couldn't find any good review papers - it's not my field at all. Perhaps a human biologist can contribute better references. $\endgroup$
    – Rik Smith-Unna
    Commented Jul 5, 2012 at 16:28
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    $\begingroup$ As additional info: The blurring of the black and white areas is not primarily caused by the movement. The eyes can only focus a very small area of the field of view. The rest is perceived in low detail. This can be visualized easily. When you move your head and keep the focus on the image, the portrait appears with low intensity. But when you focus an area next to the picture, you can see the portrait without moving your head at all. $\endgroup$
    – Mononess
    Commented Jun 4, 2013 at 18:40
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    $\begingroup$ Actually you can also see the portrait simply when looking from a distance (especially if you have myopia), without moving your head. $\endgroup$
    – Ruslan
    Commented Jul 15, 2019 at 11:21

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