WASHINGTON -- Your parents were right, don't study with the TV on.

Multitasking may be a necessity in today's fast-paced world, but new research shows that distractions affect the way people learn, making the knowledge they gain harder to use later on.

The study, in yesterday's Proceedings of the National Academy of Sciences, also provides a clue as to why it happens.

"What's new is that even if you can learn while distracted, it changes how you learn -- to make it less efficient and useful," said Russell A. Poldrack, a psychology professor at the University of California, Los Angeles.

That could affect a lot of young people. A study by the Kaiser Family Foundation last year found third-graders through 12th-graders devoted, on average, nearly 6 ¹/₂ hours per day to TV and videos, music, video games and computers.

And while he could not comment on the new report because he has not seen it, Marcel Just, director of the Center for Cognitive Brain Imaging at Carnegie Mellon University, has done research showing that when the brain tries to focus on several tasks, it does each less well than when it focuses on one.

In one study, Dr. Just asked 18 right-handed subjects to decide whether two rotating 3-D objects were the same while listening to sentences. The result was that the listening exercise reduced the brain's processing of the objects by 28 percent, and the brain's synthesis of the sentences was reduced by 50 percent in the effort to process the shapes.

As Dr. Poldrack explains it, the brain learns in two different ways.

One, called declarative learning, involves the medial temporal lobe and deals with learning active facts that can be recalled and used with great flexibility. The second, involving the striatum, is called habit learning.

For instance, Dr. Poldrack explained, in learning a phone number, you can simply memorize it, using declarative learning, and can then recall it whenever needed.

A second way to learn it is to habitually "punch it in 1,000 times. Then even if you don't remember it consciously, you can go to the phone and punch it in," he said.

Memorizing is a lot more useful, he pointed out. "If you use the habit system, you have to be at a phone to re-create the movements."

The problem, Dr. Poldrack said, is that the two types of learning seem to be competing with each other, and, when someone is distracted, habit learning seems to take over from declarative learning.

"We have to multitask in today's world, but you have to be aware of this," he said. "When a kid is trying to learn new concepts, new information, distraction is going to be bad; it's going to impair their ability to learn."

That doesn't mean he thinks a silent environment is essential -- music can help in learning because it can make the individual happier, he said. But in general, "distraction is almost always a bad thing."

What Dr. Poldrack and his colleagues did was to use brain imaging to study the parts of the brain in use when 14 people were learning. Participants were asked to predict the weather after receiving a repeated set of cues.

During part of the learning, researchers added a second task, in which participants had to keep a running mental count of high tones that they heard, thus adding an element of distraction.

The results showed that when doing single-task learning, the brain used the region associated with declarative memory, while the habit memory region was associated with dual-task learning. The dual-task learning did not affect the participants' ability to predict weather at the time, but it reduced their knowledge about the task during a follow-up session.

"In my opinion, this article represents a significant step forward in understanding the interaction between the various memory systems possessed by healthy human adults and task demands," said Chris Mayhorn, who teaches psychology at North Carolina State University and was not part of Dr. Poldrack's research team. The results, he said, suggest that at least a bit of the information is being learned even when we are distracted by a secondary task.

Dr. Mayhorn also noted that the experiment was small.