Psychologists at the University of Utah may have explained why hundreds of thousands of people missed the gorilla at the basketball practice.
(Brief pause while you watch the video. Please come back and read when you're done.)
Did you notice the gorilla that walked through in the middle? You probably did, since the title of this story warned you to keep an eye peeled. But did you notice that the curtain in the background changed color? Or that one of the players in black left the picture?
The video, posted on YouTube in various forms for years, has registered millions of hits -- and the psychologists who devised the test, led by Daniel Simons of the University of Illinois, reported that half the people they tested were so focused on the ballplayers that they actually missed the gorilla.
Now, the University of Utah psychologists have expanded on the original research, trying to quantify the "working memory capacity" that allows some people to see the unexpected while others miss it.
"Attention operates in very constrained ways at times," said Jason Watson, a University of Utah psychologist who worked on the subject with Janelle Seegmiller, a doctoral student. "There's only so much we can take in." Seegmiller, along with Watson and David Strayer, published their findings in The Journal of Experimental Psychology: Learning, Memory and Cognition.
Clearly defining "working memory capacity" could help explain why talking on a cellphone while driving is so risky for many people, or why fighter pilots need clear warnings that they're too close to the ground.
Simons and Christopher Chabris of Union College wrote a book, "The Invisible Gorilla," on our limited capacity to handle distractions.
"Those limits are largely built-in," Simons wrote in an email to ABC News. "The second, more interesting aspect of these limitations is that we are largely unaware of them. We suffer from what we call the Illusion of Attention -- we think we see and notice far more of our world than we actually do. We assume that distinctive things, like a person in a gorilla suit, will automatically capture our attention."
But they don't. Have you ever been in a crowded room, looking for a place to sit, and walked right past an old friend? Watson and Seegmiller say it's just like the gorilla -- except that your friend may be offended.
Armed with the knowledge that we can absorb only so much, automakers and designers of office software may be able to design systems to keep us alert, or help us weed out distractions. Drivers, railroad engineers, pilots and others cannot afford to miss obstacles in their way.
On the other hand, Simons notes that concentration may not always be a bad thing. Think of the surgeon who shuts out other voices when he's saving a patient, or the athlete who filters out the roar of the crowd.
"If you were constantly noticing every little thing around you," said Simons," how would you get anything done?"