Imagine a world that is completely black. You can't see a thing — unless something happens to move. You can see the rain falling from the sky, the steam coming from your coffee cup, a car passing by on the street.
This was the world that Milena Channing claimed to see, back in 2000, shortly after she was blinded by a stroke at 29 years old. But when she told her doctors about these strange apparitions, they looked at her brain scans (the stroke had destroyed basically her entire primary visual cortex, the receiving station of visual information to the brain), and told her she must be hallucinating.