This week's TED Talk: Beau Lotto: Optical Illusions show how we see
It's a fascinating presentation that will change your perception of perception. Some of his main points: What we perceive is largely based on our past experiences and expectations. Our brain often projects images of what we expect to see, and is not necessarily reflective of reality. He also says, "The brain evolved to see the world in a way that was useful in the past." We see what we need to see.
Research like this makes me very skeptical indeed of accounts of visions.