Half the People Watching the Basketball Video Don't See the Gorilla

In 1999, Daniel Simons at Harvard and Christopher Chabris at the University of Illinois at Urbana-Champaign filmed a 30-second clip of two teams of three students passing two basketballs. One team wore white shirts and one wore black. Halfway through, a graduate student in a gorilla suit walked into frame from the right, stopped at the center of the floor, beat her chest at the camera, and walked out the left side. The gorilla is on screen for nine seconds. Subjects asked to silently count the passes made by the white team frequently failed to see her. About fifty percent — "in most groups," the published paper notes — never reported the gorilla on first viewing. Many were certain, when shown the clip a second time, that someone had switched videos.

The trick exploits inattentional blindness, a documented but counterintuitive feature of how attention works. The visual cortex isn't a passive recorder; it's an active filter that throws out anything not relevant to the task you've assigned it. The gorilla doesn't slip past your eyes. Your eyes look at it; your brain treats the data as noise.

A precursor to the experiment ran in 1975, when Ulric Neisser and Robert Becklen showed subjects two superimposed video tracks of basketball games and asked them to count passes in only one. Most viewers reported nothing strange about the other game even when an unrelated woman walked through it carrying an umbrella. Simons and Chabris cleaned up the design, ran it on 228 participants, and published. The clip is now used in driver-training programs, surgical-error briefings, and TSA security training. Most viewers, on second exposure, see the gorilla immediately.