This mask can be seen either as a single face or two people kissing each other. In this example, the brain is confused which picture to perceive. This happens because contours in the picture give two different images, therefore our brain is perceiving it in two different ways. In this illusion, neither image tends to dominate, so image is flipping freely. Flipping is a mechanism of our brain to deal with ambiguity. With flipping, there are better chances of providing correct information about the environment.
While you look at this pictre, your brain is sure that these two squares A and B are different colors. But as you can see on the picture below, they are exactly the same color. What brain is doing here is making shortcuts so he can perceive reality faster than watching every single square individually and then making conclusions. This way, brain saves some time, but sometimes, these shortcuts can lead to percieving things falsly.
In the Coffer illusion you can see a series of rectangular doors panels, but after a while, your brain will "flip" that image - you will see 16 circles. There is no wrong perception because contours are distributed to make the picutre ambiguous. In this illusion rectangulars dominates, probably for the reason that rectangles are much more common shapes in our environment than circles, so our brain "favors" them.
In this illusion, we see the same face that appears to be female's when the tone is lighter and male's when the tone is darker. This illusion works because changes of tone influence the contrast of pictures. Based on contrast, our brain will decide weather that face is male or female. Of course, there is many more other cues for our brain to determine a sex, but when other usual cues are lacking, contrast can be deciding factor.
