From The Brain Book
The brain has rules it applies to incoming information to make sense of it quickly. If we hear a voice and at the same time see a mouth moving, we assume the voice comes from the mouth. But this is only a best guess and can be wrong.
Something strange happens when we look at ambiguous figures, as in the picture on the right. The input to the brain is the same, but what we see flips from one thing to another.
It is impossible to see which shape is the main one, so the brain tries one way of seeing it, then another. You see both images (of a vase and two faces, in this case) but you can never see both simultaneously.
my wife says my mind is twisted because i always see both images at the same time. but cant see those pix that have all the mixed patterns and an image will appear if you stare at it long enough.