Nina Ahuja has a winning smile, three screen names and 300 buddies on her AOL Instant Messenger list. On a typical school night, she does...

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Nina Ahuja has a winning smile, three screen names and 300 buddies on her AOL Instant Messenger list.

On a typical school night, she does her homework with the Style Network playing on TV and Missy Elliott rapping from her iPod mini. A barrage of instant messages and mobile phone calls punctuates the cacophony.

The only thing that jars her is the thought of unplugging anything — even for a day.

“I’d probably go crazy. I’d be mentally unstable,” says Nina, 16. An instant-messaging user since the second grade, she has been glued to a cellphone for four years and barely scrapes by on 1,000 minutes of talk time per month.

Nearly a third of kids in grades 3 through 12 regularly multitask while doing homework, according to a report by the Kaiser Family Foundation.

What does this fractured focus mean long-range?

Small studies suggest distractions impair learning. Yet scores on IQ tests keep rising, and author Steven Johnson credits pop culture. Juggling communications and making sense of software, chaotic video games and fast-paced, ambiguous movies (“Memento”) and TV shows (“The West Wing”) exercises “cognitive muscles,” he writes in “Everything Bad Is Good For You” (Riverhead Trade, $14).

Does multitasking just come naturally to kids? Or is it a skill that can be sharpened with practice?

No, and maybe.

The eminent scholar Yogi Berra once observed that you can’t think and hit a baseball at the same time.

Neuroscientists agree. The brain can’t concentrate on two things at once.

Multitasking is a misnomer: People toggle between tasks, says Edward Hallowell, an expert on attention deficit hyperactivity disorder.

And the best togglers seem to be 20-somethings, not children, based on research by the University of Illinois. People ages 7 to 82 were asked to switch between two memory tasks in some simple numeric experiments. Both ends of the age spectrum did poorly, with young kids faring worst, says Art Kramer, a professor of neuroscience and psychology.

Kramer says the brain’s prefrontal cortex develops fully in the 20s. Performance declines after 60.

Multitasking might make students dumber when it comes to writing essays or grasping Shakespeare, according to a study by the University of California, Los Angeles. Functional magnetic resonance imaging, which tracks blood flow, monitored brain activity in 14 subjects for two sets of similar experiments. During one round they also were asked to keep track of beeping sounds, to simulate multitasking.

The experiments activated separate parts of the brain, scans showed.

Focused tasks tapped the hippocampus, a region responsible for deep learning and subtle ideas. For multitasking, the basal ganglia took over. That is the brain’s autopilot, for behaviors that become second nature.

The subjects, all in their 20s, later struggled to recall what they had learned while multitasking.

“When people learned under multitasking conditions, their learning was not as flexible,” says psychology professor Barbara Knowlton, co-author of the UCLA study. “They could do simple associations, but when they were asked to probe deeper and had to use information that might replicate real learning, they really suffered.”

Music can boost mood, Knowlton says. But singing along, like watching TV or answering instant messages, is distracting. “It might give you a superficial sense that you’re learning,” Knowlton says.

Aside from screen names, Ahuja’s memory is “horrible.” She says there is little time for reading or, surprisingly, for close friendships.

“What I regret most is not having a childhood. I don’t feel I ever had a time in my life past 7 where I could just play with dolls, or just play with friends. I didn’t go out and ride a bike. I was too consumed with this stuff.”