RENEE MONTAGNE, HOST:

This is MORNING EDITION, from NPR News. I'm Renee Montagne.

STEVE INSKEEP, HOST:

And I'm Steve Inskeep. Throughout this morning, we're hearing more about Pope Benedict's resignation. Vatican Radio has released tape of the Pope discussing his decision in Latin.

(SOUNDBITE OF VATICAN RADIO BROADCAST)

POPE BENEDICT XVI: (Latin spoken)

INSKEEP: Now, a translation of the Pope's written statement quotes him saying that "in today's world, his job requires strength of body and mind - strength, which in the last few months, has deteriorated in me." The phrase in today's world is noteworthy. For centuries, before this day, many pontiffs grew old and infirm, but none resigned, for more than 600 years.

Now today In Your Health, we have a story that teaches us more about how we see the world. Here's NPR's Alix Spiegel.

ALIX SPIEGEL, BYLINE: This story about how you see the world begins with a group of people who are expert at looking: the professional searchers known as radiologists.

TRAFTON DREW: If you watch radiologists do what they do, I was absolutely convinced that they are like superhuman.

SPIEGEL: This is a Harvard researcher named Trafton Drew. And about three years ago, he started watching radiologists do their work. For hours, he would stand in their dark viewing rooms, in awe that they could so easily see in the images before them things that to him were simply invisible.

DREW: These tiny little nodules that I can't even see when people point to them. You know, they're just in a different world in terms of finding this very, very hard to fine thing.

SPIEGEL: But radiologists still sometimes fail to see important things. And Drew, who studies visual attention at Harvard, wanted to understand more. Now, because of his work in this field, Drew was naturally familiar with one of the most famous studies in the field of attention research: The Invisible Gorilla experiment.

(SOUNDBITE OF A VIDEO)

DANIEL SIMONS: This video is from research by Daniel Simons and Christopher Chabris...

SPIEGEL: That's psychologist Daniel Simons introducing the video that's used in the study. Before the video begin, viewers are told that they will see two groups of kids passing a basketball back and forth. And they're told to do one thing and one thing only.

(SOUNDBITE OF A VIDEO)

SIMONS: Count how many times the players wearing white pass the ball.

SPIEGEL: Drew says this is actually quite hard because the players are constantly moving around.

DREW: You'd have to really think, you know, you're counting in your head - one, two, three, four, five. And as you're counting that, a gorilla wanders onto the screen, looks straight ahead, beats his chest and walks off the screen - 19, 20.

SPIEGEL: Then the viewer is asked...

(SOUNDBITE OF A VIDEO)

SIMONS: Did you see the gorilla?

DREW: Sounds ridiculous, right? There's a gorilla on the screen, of course you're going to see it. But 50 percent of people miss the gorilla.

SPIEGEL: This is because when you ask someone to perform a challenging task, without realizing it, their attention narrows and can block out other things - even huge, hairy gorillas that appeared directly in front of them.

Which brings us back to the expert lookers: the radiologists. Trafton Drew wondered if somehow being so well trained in searching would make them immune to missing large, hairy gorillas.

DREW: You might expect that because they're experts, they would notice if something unusual was there.

SPIEGEL: And so, he took a picture of a man in gorilla.

DREW: It's not an actual gorilla. It's a man in a gorilla suit. But he's shaking his fist angrily.

SPIEGEL: And he superimposed that image on a series of the slides that radiologists typically look at when they're searching for cancer. Then he asked a bunch of radiologists to review the slides for cancerous nodules. He wanted to see if they would notice a gorilla the size of a matchbook glaring angrily at them from inside the slide - but they didn't.

DREW: Eighty-three percent of the radiologists did not see the gorilla.

SPIEGEL: Now, it wasn't because that the eyes of the radiologists didn't happen to fall on the large angry gorilla. The problem was in a way their brains had framed what they were doing. They were looking for cancer, not gorillas. And so...

DREW: They look right at it, but because they're not looking for a gorilla, they don't see that it's a gorilla.

SPIEGEL: In other words, what we're focused on filters the world around us so aggressively that it literally shapes what we see. And so, Drew says, we need to think really carefully about the instructions that we give to professional searchers, like radiologists or people looking for terrorist activity. Because what we tell them to look for will in part determine what they see and don't see.

Drew and his co-author Jeremy Wolfe are doing more studies, looking at how to help radiologists and other people see both visually and cognitively the things that hide in plain sight.

Alix Spiegel, NPR News, Washington.

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