Optical illusions are caused by the fact that we see as much with our brains as with our eyes. What we expect to see is hard wired both biologically and by experience. In the above videos we can accept that we are looking at a reflection and yet is can also appear to be the right way up most of the time because that is how we generally view scenes.
Now have a look at the three images below:
Normal view (well, as normal as can be!)
Upside Down view
The actual image used for the upside down view (Eyes and mouth had been rotated through 180 degrees)
The first and second photos look (relatively) normal and recognisably me. However, the second picture is not the first one turned upside down. The third picture is the one actually used to create the inverted second photo. The gross distortion in photo three was created by cutting out the eyes and mouth in a photo editor and turning them upside down (I also flipped the mouth horizontally to make it fit better). You can see this most clearly if you look at the lips in photos one and two.
When the third image is inverted, the eyes and the mouth are actually the right way up. Our brain interprets the second photo and we are fooled into seeing an apparently normal upside down face.
If you are able to edit photos, have some fun with your friends by trying out the face inversion with eyes and mouth.
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