For an expert radiologist a computed tomography (CT) scan provides a wealth of information, and helps to establish diagnoses such as lung cancer. Because forming a diagnosis often requires finding anomalies in a scan, you would think that radiologists are best equipped to detect any anomaly.
However, a recent study by a team of scientists at the Visual Attention Lab of Harvard Medical School shows that even experts whose job it is to visually search complex images can sometimes miss information that is literally right in front of their eyes.
For an ordinary person CT scans do not provide much information, even though it is undeniably cool to have a peek at your bones, brain or lungs. Take a look at the lung scan at the top of this article and imagine you are a radiologist tasked with finding lung nodules that appear as small white circles and can be indicative of lung cancer.
Chances are, you didn’t find any, because you are not sure what nodules look like. But did you notice anything else strange? If not, look closely at the top right corner. Yes, that’s a gorilla.
This is essentially the same task researchers asked 24 radiologists to perform. The gorilla image was squirrelled away in a stack of slices of a lung CT, a standard lung cancer screening approach. 20 of the radiologists did not notice the gorilla while scrolling through the stack, even though eye tracking revealed that 12 of them had looked straight at it. Also, the gorilla was approximately the size of a match box, 48 times larger than the average lung nodule.
So, what is going on here?
The phenomenon is not new to visual attention researchers and is known as “inattentional blindess”. When you are engaged in an attention demanding visual task, it is easy to miss even highly abnormal and visually salient features of the environment. The first study that drew attention to this fascinating quirk of cognition featured a video of a basketball game and had something to do with gorillas as well (but I’m not giving away spoilers).
The research team designed this CT scan study to see whether expertise in the primary task mitigates the effect. After all, if you are more skilled in looking for nodules, there should be more attention left over to notice other things, too.
To an extent that is indeed the case, because the 24 naive viewers whom the researchers tasked with the same lung nodule search after a brief how-to, performed worse. None of them noticed the gorilla, that’s how preoccupied they were with the main task.
So, should we be worried about the competency of said radiologists? Not at all. As the paper1 points out, “the message of the present results is that even this high level of expertise does not immunize against inherent limitations of human attention and perception.”
After all, the experts did reasonably well in the challenging primary task, but they were not looking for gorillas.
Inattentional blindness reminds us that our perception of the world is not as perfect and complete as we think it is. Visual attention science like this tells us why we shouldn’t drive and talk on the phone at the same time, and why it’s easy for a graphic designer to miss a glaring typo on the cover of a magazine.
Perhaps this will be a consolation next time you find your keys in the spot “you looked a minute ago and they weren’t there”.