The Cheerleader Effect: Scientific American

"All of these visual illusions demonstrate that what we 'see' is not always a simple or direct reflection of what is right in front of us. Instead, what we see depends on both the physical stimulus coded by our visual systems (what cognitive scientists refer to as bottom up processing), and a blend of contextual information, expectations, and prior knowledge (known as top down processing)."

So much for "seeing is believing".

via Arts & Letters Daily