Over 400,000 people visited Farnam Street last month to learn how to make better decisions, create new ideas, and avoid stupid errors. With more than 98,000 subscribers to our popular weekly digest, we've become an online intellectual hub. To learn more about we what do, start here.

The (Really) Invisible Gorilla

invisible gorilla

Inattentional blindness is the phenomenon of not being able to see things that are actually there. This concept was popularized in 2010 book The Invisible Gorilla: How Our Intuitions Deceive Us by Daniel Simons and Christopher Chabris.

The best known experiment demonstrating inattentional blindness involves a basketball. Simons and Chabris gave subjects a very simple task: try and keep track of how many times the basketball was passed between players. While the ball was being tossed someone in a gorilla suit walks across the screen, in plain view. What they discovered was unexpected. Few people noticed the gorilla because they were so focused on counting the passes. When we pay close attention to one thing, we fail to notice other things.

A recent article by Wray Herbert covers some new research that this perceptual quirk can have serious, even life-threatening, implications.

Three psychological scientists at Brigham and Women’s Hospital in Boston –Trafton Drew, Melissa Vo and Jeremy Wolfe — wondered if expert observers are also subject to this perceptual blindness. The subjects in the classic study were “naïve” — untrained in any particular domain of expertise and performing a task nobody does in real life. But what about highly trained professionals who make their living doing specialized kinds of observations? The scientists set out to explore this, and in an area of great importance to many people — cancer diagnosis.

Radiologists are physicians with special advanced training in reading various pictures of the body — not just the one-shot X-rays of the past but complex MRI, CT and PET scans as well. In looking for signs of lung cancer, for example, radiologists examine hundreds of ultra-thin CT images of a single patient’s lungs, looking for tiny white nodules that warn of cancer. It’s these expert observers that the Brigham and Women’s scientists chose to study.

They recruited 24 experienced and credentialed radiologists — and a comparable group of naïve volunteers. They tracked their eye movements as they examined five patients’ CT scans, each made up of hundreds of images of lung tissue. Each case had about ten nodules hiding somewhere in the scans, and the radiologists were instructed to click on these nodules with a mouse. On the final case, the scientists inserted a tiny image of a gorilla (an homage to the original work) into the lung. They wanted to see if the radiologists, focused on the telltale nodules, would be blind to the easily detectable and highly anomalous gorilla.

The gorilla was miniscule, but huge compared to the nodules. It was about the size of a box of matches — or 48 times the size of a typical nodule. It faded in and out — becoming more, then less opaque — over a sequence of five images. There was no mistaking the gorilla: If someone pointed it out on the lung scan and asked, What is that? — everyone would answer: That’s a gorilla.

After they were done scrolling through the images as much as they wanted, the scientists asked them: Did that last trial seem any different? Did you notice anything unusual on the final trial? And finally: Did you see a gorilla on the final trial? Twenty of the 24 radiologists failed to see the gorilla, despite scrolling past it more than four times on average. And this was not because it was difficult to see: When shown the image again after the experiment, all of them saw the gorilla. What’s more, the eye-tracking data showed clearly that most of those who did not see the gorilla did in fact look right at it.