I love optical illusions. First, they're fun. Second they give us insight into how we think, which is pretty nifty. Pareidolia is a personal favorite, of course, but I also like simple illusions that fool us.
A new study* of how we perceive things shows that context is critical. We know that of course, but what these researchers found is that under some circumstances, our brains make things up as filler more often depending on context.
For example, a vague or faint background allows our brains to fill in the blanks more readily. If you see something, and the background is indistinct, your brain is more likely to add in details that simply weren't there than if the background is very distinct.
I wonder if this might explain seeing ghosts or aliens in dark rooms (along with my own personal explanation). A vague shape is prime for this effect, and coupled with pareidolia I wonder if this doesn't very well explain the vast majority of paranormal phenomena.
*Search for the title "Filling-In and Suppression of Visual Perception from Context: A Bayesian Account of Perceptual Biases by Contextual Influences".
Tip o' the ectoplasmic beanie to BABLoggee Bret Hall.