Headspace Discovery

There's a famous optical illusion. Young woman, or old hag.

When I look at a picture, I know how to manipulate my eye muscles so that the picture is blurry. I know how to manipulate my eye muscles so that I see double. As far as I know, that's a completely physiological response. It involves my eye.

I also know how to shift my perception so that I see an old hag. Now, I see a young woman. Now the hag. What is it that I'm changing? A perception muscle? A filter for pattern recognition? And is my experience of moving easily from one way of looking to another idiosyncratic? Do some people just simply perceive both at the same time?

And, more interestingly, do people generally think of there being a way of perceiving that can be changed? My sense is that most people think that they just see.