The Gorilla Lurking Where We Can’t See It; A study of radiologists shows that when we pay careful attention to one thing we become literally blind to others—even startling ones like gorillas

August 9, 2013, 8:34 p.m. ET

The Gorilla Lurking Where We Can’t See It

ALISON GOPNIK

Imagine that you are a radiologist searching through slides of lung tissue for abnormalities. On one slide, right next to a suspicious nodule, there is the image of a large, threatening gorilla. What would you do? Write to the American Medical Association? Check yourself into the schizophrenia clinic next door? Track down the practical joker among the lab technicians? In fact, you probably wouldn’t do anything. That is because, although you were staring right at the gorilla, you probably wouldn’t have seen it. That startling fact shows just how little we understand about consciousness.In the journal Psychological Science, Trafton Drew and colleagues report that they got radiologists to look for abnormalities in a series of slides, as they usually do. But then they added a gorilla to some of the slides. The gorilla gradually faded into the slides and then gradually faded out, since people are more likely to notice a sudden change than a gradual one. When the experimenters asked the radiologists if they had seen anything unusual, 83% said no. An eye-tracking machine showed that radiologists missed the gorilla even when they were looking straight at it.

This study is just the latest to demonstrate what psychologists call “inattentional blindness.” When we pay careful attention to one thing, we become literally blind to others—even startling ones like gorillas.

In one classic study, Dan Simons and Christopher Chabris showed people a video of students passing a ball around. They asked the viewers to count the number of passes, so they had to pay attention to the balls. In the midst of the video, someone in a gorilla suit walked through the players. Most of the viewers, who were focused on counting the balls, didn’t see the gorilla at all. You can experience similar illusions yourself at invisiblegorilla.com. It is an amazingly robust phenomenon—I am still completely deceived by each new example.

You might think this is just a weird thing that happens with videos in a psychology lab. But in the new study, the radiologists were seasoned professionals practicing a real and vitally important skill. Yet they were also blind to the unexpected events.

In fact, we are all subject to inattentional blindness all the time. That is one of the foundations of magic acts. Psychologists have started collaborating with professional magicians to figure out how their tricks work. It turns out that if you just keep your audience’s attention focused on the rabbit, they literally won’t even see what you’re doing with the hat.

Inattentional blindness is as important for philosophers as it is for radiologists and magicians. Many philosophers have claimed that we can’t be wrong about our conscious experiences. It certainly feels that way. But these studies are troubling. If you asked the radiologist about the gorilla, she’d say that she just experienced a normal slide in exactly the way she experienced the other slides—except that we know that can’t be true. Did she have the experience of seeing the gorilla and somehow not know it? Or did she experience just the part of the slide with the nodule and invent the gorilla-free remainder?

At this very moment, as I stare at my screen and concentrate on this column, I’m absolutely sure that I’m also experiencing the whole visual field—the chair, the light, the view out my window. But for all I know, invisible gorillas may be all around me.

Many philosophical arguments about consciousness are based on the apparently certain and obvious intuitions we have about our experience. This includes, of course, arguments that consciousness just couldn’t be explained scientifically. But scientific experiments like this one show that those beautifully clear and self-evident intuitions are really incoherent and baffling. We will have to wrestle with many other confusing, tricky, elusive gorillas before we understand how consciousness works.