Illusion

This really bothers me!  How does this work?  How is it that our eyes, like photography, "develop" a positive image from a negative?

Contrast affects perception ...

Here is an interesting illusion as created by increasing and decreasing contrast. "The Illusion of Sex" (2009 Richard Russell for Best Illusion of the Year Contest; 3rd Place) illustrates, by use of photographs, our perception of what appears female and what appears male. The increase in contrast makes the same image of the same person's face appear more feminine. According to Scientific American Mind magazine, this would also explain "... why females in many cultures darken their eyes and mouths with cosmetics: a made-up face looks more feminine than a fresh face."