Human Perception and the Invisible Gorilla

When they taught together at Harvard in the late 1990s, psychologist Daniel Simons and his student Christopher Chabris got an idea for a new experiment testing how the brain processes visual information. Their 60-second test was outrageously simple: it required only that you watch people passing basketballs. (I’m going to reveal the secret of the test below, so you might want to take the original test before continuing.)

Simons and Chabris had little clue that their experiment would become one of the most famous brain quizzes of the past half-century. Media outlets around the world publicized the test, and NBC aired a Dateline feature about it. The study was even mentioned on a 2001 episode of CSI. Since then, Simons has updated the experiment, and on July 12 he released the results in a new journal called i-Perception. (You can take the new test here. I’m also going to give away the secrets of this one.)

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In the original Simons-Chabris test, subjects were asked to watch a video of six students passing basketballs. Three were dressed in black and three in white. Viewers were supposed to count the number of times the players in white passed the ball. During the video, a woman in a full-body gorilla suit walked into the center of the frame, pounded her chest and then walked off. Shockingly, about half the people who took the test — in countless airings of the video all over the world — did not notice the woman in the gorilla suit. Some who didn’t see the woman in the suit protested that the video had been rigged. People who did see her were incredulous: how could so many miss something so obvious? Simons and Chabris had stumbled onto a basic lapse in human visual perception: “inattentional blindness,” the failure to see something conspicuous when focusing attention on something else.

In the new i-Perception paper, Simons (this time without Chabris) updates the famous gorilla experiment. The new video also features students divided into two teams, passing basketballs. We are instructed again to count the number of passes by players in white. The gorilla is back. But there are two new visual tricks: a curtain behind the players changes color during the video, and one of the players in black walks out of the frame.

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Simons, now a professor of psychology at the University of Illinois, got 76 test subjects to watch the new video in the student-union building on campus. Of the small sample, 12 students were excluded for various reasons, so Simons ended up with just 64. Of those, 23 said they were familiar with the gorilla experiment; predictably, all 23 said they saw the gorilla in the new video. But only one of those 23 noticed both the curtain changing color and the player walking out. Among the 41 who were unfamiliar with the original experiment, no one noticed the color change or the player’s departure. About half (18 of the 41) saw the gorilla, reconfirming the results of the original test.

The professor concludes from these results that even when you expect something unexpected to happen, you may not notice it — especially if you are looking for someone in a gorilla suit. Which is probably true, but the unexpected events you’re supposed to notice — the curtain and the player walking out — are not simply unexpected but also completely random. Compared with something like a gorilla, the new tricks seem like small matters of set design. It would be strange if you did notice them, especially if you’re supposed to be counting ball passes.

Earlier this year, Crown published a book by Simons and Chabris called The Invisible Gorilla: And Other Ways Our Intuitions Deceive Us. The book is firmly in the Malcolm Gladwell school: it uses anecdotes and social-psychology data to spin the plain world you know into a wonderment of surprising new insights. The authors are so confident of their interpretation of their anecdotes and their data, they say in the introduction that the book can help you “see through the veils that distort how we perceive ourselves and … connect you — for perhaps the first time — with reality.”

I feel fairly connected with reality already, so I wasn’t shocked to find that the book overpromised. For instance, in a discussion of the many instances in which religious believers see the Virgin Mary in everyday objects (a grilled-cheese sandwich, a salt stain on a Chicago underpass), the authors say the misperceptions are evidence that “your brain can be activated by images that only vaguely resemble what they’re tuned for.” But isn’t it possible that these Virgin Mary sightings are merely evidence of intense religious yearning? If so, why would we assume the phenomenon applies broadly to all of us?

Similarly, Simons and Chabris use a George W. Bush misstatement about his immediate reaction to 9/11 as an example of how memory can be a mere “illusion.” Bush said in December 2001 that he had watched on TV as the first plane hit the World Trade Center, which would have been impossible since no video of the first plane crash surfaced for months. Maybe Bush was mistaken, and maybe that means memory can be illusory, but isn’t it just as likely that the President was spinning the story on the fly for political effect?

For the record, I did not notice either visual trick in the new video, so maybe I’m just bitter. I didn’t watch the original video before I knew about the gorilla, but I expect I would have been in the half who didn’t see it. I’m not sure if that makes me less perceptive — or more discerning, more focused. Sometimes, it’s better not to see the gorilla.

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