Inattentional Blindness (The Invisible Gorilla Experiment)

A gorilla nobody saw

In the famous version, you watch a short video of two teams passing basketballs, one in white shirts and one in black, and count how many passes the white team makes. It takes real concentration: the players weave around each other and pass quickly.

Partway through, a person in a gorilla suit walks into the middle of the players, turns to the camera, thumps their chest and walks off. In Daniel Simons and Christopher Chabris's experiment, that took about nine seconds, in full view. Afterwards, viewers were asked whether they'd noticed anything unusual. Across all the versions they tested, 46% of 192 observers hadn't (Simons & Chabris, 1999). Shown the clip again without the counting task, people find the gorilla impossible to miss, and many can't believe they looked straight at it.

This is inattentional blindness: failing to see something fully visible because your attention is busy elsewhere.

Try the lab version

The gorilla video is under copyright, but the demo above follows an older, simpler method from Arien Mack and Irvin Rock's research. A cross flashes for a fifth of a second and you judge which arm is longer. The first three flashes are just the cross. On the fourth, a small shape appears right beside it, on screen exactly as long as the cross and close to where you're looking. Many people never see it (Mack & Rock, 1998). The results from this site's visitors appear once you've tried it; only first attempts count, because once you know about the shape, you'll be looking for it.

Looking isn't the same as seeing

It feels as though anything in front of your eyes is something you see. Inattentional blindness shows that isn't so. Becoming aware of something, in a way you can later report, depends on attention, and attention is limited: the task you're doing uses most of it.

Counting passes means following one team and actively ignoring the other. That filtering appears to suppress things that don't fit what you're looking for, rather than just leaving them unattended. A gorilla isn't hard to see; it just isn't a white-shirted player passing a ball, so it gets filtered out with everything else that isn't. The light from it reaches your eyes and early visual processing handles it normally. What fails is the later step that turns it into something you're aware of.

Different from change blindness

Most illusions on this site make you see something that isn't there, such as a contour or a colour. Inattentional blindness is the reverse: you fail to see something that is. It's closely related to change blindness, where large changes go unnoticed, but there's an important difference. Change blindness needs the change to be hidden, by a flicker or an interruption. Inattentional blindness needs nothing hidden at all; attention alone does it.

Experts aren't immune

Trafton Drew, Melissa Võ and Jeremy Wolfe hid a picture of a gorilla, many times larger than a typical nodule, in a set of lung CT scans and asked radiologists to look for cancerous nodules. 83% of the radiologists didn't report the gorilla, and eye tracking showed that most of those who missed it had looked directly at where it was (Drew, Võ & Wolfe, 2013). Expertise makes people very good at finding what they're looking for; it doesn't make them notice what they aren't.

Where the idea came from

Simons and Chabris built on earlier work by Ulric Neisser, who in the 1970s showed people two overlapping videos and found that those following one often missed odd events in the other, including a woman with an umbrella walking through a ball game. What made the 1999 study famous was how blatant its unexpected event was: a gorilla thumping its chest leaves no room to argue that it was too small or too brief to see. Chabris and Simons later wrote a popular book, The Invisible Gorilla, about how much we overestimate what we notice.