Look closely at the faces shown in the drawing. Initially, you will see the face of an angry man on the left and a woman with a serious face on the right. Now, get away from the screen, enough for the image to look distant and blurry. You must separate at least 3 to 4 meters. If you don't have enough space in front of the computer, try printing the image and hanging it on the wall. You can also reduce the image with the zoom of your browser until you observe the effect.
As you walk away you will see that the face on the left becomes the woman with a serious face and the face on the right becomes the angry man. Amazing, right?
This impressive illusion was created by Aude Oliva and Philippe Schyns and shows the ability of the visual system to separate information from different spatial frequency channels. In the image on the right the high spatial frequencies (HSF) represent a woman with a neutral facial expression mixed with the low spatial frequency (LSF) information that represents the face of the angry man.