Look carefully at the lung scan below. Do you notice anything strange about it?
If you didn't spot the dancing gorilla in the top right hand corner, fear not - you're not alone.
A study showed 83% of 24 radiologists (who were asked to search for cancer nodules) missed it too.
"On the final scan, a gorilla about 48 times the size of an average nodule was placed in the upper right hand quadrant."
The image - an X-ray computed tomography (CT) scan - is part of an upcoming study by psychological scientists at Boston's Brigham and Women's Hospital, due to be published in the journal Psychological Science.
Trafton Drew, a post-doctroral fellow at the Brigham and Women's Hospital Visual Attention Lab told National Geographic: "It's a vivid example that looking at something and seeing it are different.
"You can put your eyes on something, but if you're not looking for it, you're functionally blind to it."
A comparable group of untrained volunteers also took part in the study, with none of them spotting the gorilla (compared to four out of the 20 radiologists who did see it).
Eye-tracking data also revealed most of those who did not see the gorilla did in fact look straight at it.
The Association for Psychological Science explains: "To their credit, the trained radiologists did detect the anomaly more often than the naïve volunteers. Indeed, none of the untrained volunteers reported seeing the gorilla, so it does seem that experts are somewhat less prone to this form of blindness. It's probably because their full attentional capacity is not consumed by the primary task, which they are accustomed to."
The study comes on the back of the Invisible Gorilla phenomena, sparked by a book of the same name, which examined human perception and "inattentional blindness".
For the research, psychologists Dr Daniel Simons and Dr Christopher Chabris asked study participants to watch a fast-paced video in which a group of people pass a basketball and to count how many times certain people tossed the ball. Actually, you try it (watch the video below).
Did you notice the gorilla in the room? In the middle of the video, a person in a gorilla suit walks into the frame--a seemingly obvious intrusion that was noticed by only about half of the participants in Dr. Simons' study. It wasn't that the participants weren't paying attention but their selective attention had caused inattentional blindness.
Blogging for the Huffington Post in January, science writer Wray Herbert explains: "What the invisible gorilla study shows is that, if we are paying very close attention to one thing, we often fail to notice other things in our field of vision - even very obvious things.
"We all love these quirks of human perception. It's entertaining to know that our senses can play tricks on us. And there's no doubt the extent of most people's familiarity with this psychological phenomenon."
He adds: "But what if this perceptual quirk has serious implications - even life threatening implications?"
Drew does not see it as necessarily bleak news however, pointing out to CBS in a further email interview: "Radiologists are amazingly good at finding cancer, but that does not mean they see everything.
"One reason that they are so good at detecting cancer might be that they are really tightly focusing their attention on the task at hand. The consequence of focusing your attention really tightly is that you may be prone to missing things which may be pretty obvious in retrospect."