You don’t see what you are not looking for! "Another follow-up study by Trafton Drew asked radiologists to read a CT scan of the lungs. He placed an obvious picture of a gorilla in a dark part of the scan. It is clearly visible, and yet 82 percent of trained radiologists missed the gorilla. Radiologists, while experts at reading such scans, are still humans. They’re not looking for a gorilla. They’re looking for tumors and other known pathology, and you don’t see what you are not looking for." The Skeptics' Guide to the Universe Steven Novella