"He then asked a bunch of radiologists to review the slides of lungs for cancerous nodules. He wanted to see
if they would notice a gorilla the size of a matchbook glaring angrily at them from inside the slide.
But they didn't: 83 percent of the radiologists missed it, Drew says.
This wasn't because the eyes of the radiologists didn't happen to fall on the large, angry gorilla.
Instead, the problem was in the way their brains had framed what they were doing.
They were looking for cancer nodules, not gorillas. "They look right at it, but because they're
not looking for a gorilla, they don't see that it's a gorilla," Drew says.
In other words, what we're thinking about — what we're focused on — filters
the world around us so aggressively that it literally shapes what we see"
http://www.npr.org/blogs/health/2013/02/11/171409656/why-even-radiologists-can-miss-a-gorilla-hiding-in-plain-sight