Researcher Trafton Drew at Brigham and Women's Hospital in Boston inserted a big picture of a gorilla in images of lungs. Highly skilled radiologists were asked to check the images for lung nodules -- small patches of tissue that could indicate lung cancer.
About 80 percent of the radiologists did not see the gorilla because they were so focused on the task of looking for the nodules, Drew said.
"You tend to see what you're looking for and you're often effectively blinded to all things that you're not expecting," Drew said in a statement.
Drew said everyone does it, and it's something to consider the next time you're cruising through an intersection. "Did you see the boy on the bike?" he asked.
The study was published in the journal Psychological Science.