₁You’re at a party. Music is playing. Glasses are clinking. Dozens of conversations are driving up the decibel level. Yet amid all those distractions, you can zero in on the one conversation you want to hear.

₂This ability to hyper-focus on one stream of sound amid a cacophony of others is what researchers call the “cocktail-party effect.” Now, scientists at the University of California in San Francisco have pinpointed where that sound-editing process occurs in the brain—in the auditory cortex just behind the ear, not in areas of higher thought. The auditory cortex boosts some sounds and turns down others so that when the signal reaches the higher brain, “it’s as if only one person was speaking alone,” says principle investigator Edward Chang.

₃These findings, published in the journal Nature last week, underscore why people aren’t very good at multitasking—our brains are wired for “selective attention” and can focus on only one thing at a time. That innate ability has helped humans survive in a world buzzing with visual and auditory stimulation. But we keep trying to push the limits with multitasking, sometimes with tragic consequences. Drivers talking on cellphones, for example, are four times as likely to get into traffic accidents as those who aren’t.

₄Many of those accidents are due to “inattentional blindness,” in which people can, in effect, turn a blind eye to things they aren’t focusing on. Images land on our retinas and are either boosted or played down in the visual cortex before being passed to the brain, just as the auditory cortex filters sounds, as shown in the Nature study last week. “It’s a push-pull relationship—the more we focus on one thing, the less we can focus on others,” says Diane M. Beck, an associate professor of psychology at the University of Illinois.

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₅That people can be completely oblivious to things in their field of vision was demonstrated famously in the “Invisible Gorilla experiment” devised at Harvard in the 1990s. Observers are shown a short video of youths tossing a basketball and asked to count how often the ball is passed by those wearing white. Afterward, the observers are asked several questions, including, “Did you see the gorilla?” Typically, about half the observers failed to notice that someone in a gorilla suit walked through the scene. They’re usually flabbergasted because they’re certain they would have noticed something like that.

₆“We largely see what we expect to see,” says Daniel Simons, one of the study’s creators and now a professor of psychology at the University of Illinois. As he notes in his subsequent book, “The Invisible Gorilla” (co-authored with Christopher Chabris), the more attention a task demands, the less attention we can pay to other things in our field of vision. That’s why pilots sometimes fail to notice obstacles on runways and radiologists may overlook anomalies on X-rays, especially in areas they aren’t scrutinizing.

₇And it isn’t just that sights and sounds compete for the brain’s attention. All the sensory inputs vie to become the mind’s top priority.

₈That’s the real danger of distracted driving, experts say. “You regularly hear people say as long as your hands are on the wheel and your eyes are on the road, you’re fine. But that’s not true,” Mr. Simons says.

₉Studies over the past decade at the University of Utah show that drivers talking on hands-free cellphones are just as impaired as those on hands-held phones because it is the conversation, not the device, that is draining their attention. Those talking on any kind of cellphone react more slowly and miss more traffic signals than other motorists.

₁₀“Even though your eyes are looking right at something, when you are on the cellphone, you are not as likely to see it,” says David Strayer, a psychology professor and lead researcher. “Ninety-nine percent of the time, it’s not that critical, but that 1% could be the time a child runs into the street,” he adds.

₁₁Dr. Strayer’s studies have also found that talking on a cellphone is far more distracting than conversing with a passenger—since a passenger can see the same traffic hazards and doesn’t expect a steady stream of conversation as someone on a cellphone does. Listening to the radio, to music or to a book on tape also isn’t as distracting, because it doesn’t require the same level of interaction as a conversation. But Mr. Simons notes that even drivers may miss some details of a book on tape if their attention is focused on merging or other complex driving tasks.

₁₂Some people can train themselves to pay extra attention to things that are important—like police officers learn to scan crowds for faces and conductors can listen for individual instruments within the orchestra as a whole.

2.5%: The percentage of people who can multitask efficiently. Many more people only think they can.

₁₃And the Utah researchers have identified a rare group of “super-taskers”—an estimated 2.5% of the population—who seem able to attend to more than one thing with ease.

₁₄Many more people think they can effectively multitask, but they are really shifting their attention rapidly between two things and not getting the full effect of either, experts say.

₁₅Indeed, some college professors have barred students from bringing laptop computers to their classrooms, even ostensibly to take notes. Dr. Beck says she was surprised to find that some of her students were on Facebook during her lectures—even though the course was about selective attention.

₁₆Still, she doesn’t plan to crack down. “I just explained that doing Facebook in class means you will not learn as much, which will have consequences on the exam,” she says.

₁₇Clearly, it is easier to combine some tasks than others. “Not all distractions are the same,” says Dr. Strayer. Things like knitting, cleaning and working out can be done automatically while the mind is engaged elsewhere. But doing homework and texting simultaneously isn’t possible. (Sorry, kids).

₁₈Even conversing and watching TV is difficult. “Just try conversing with your wife while watching football. It’s impossible,” jokes Mr. Simons.