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 distractions affect the way people learn, making the knowledge they gain harder to use later on.
The study, in Monday'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 6.5 hours per day to TV and videos, music, video games and computers.
As 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, in learning a phone number you can simply memorize it, using declarative learning, and can then recall it whenever needed, Poldrack explained.
A second way to learn it is by habit, "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 recreate the movements," he said.
The problem, 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 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 where 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 later.
"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," commented Dr. Chris Mayhorn, who teaches psychology at North Carolina State University.
The results suggest that at least a bit of the information is being learned even when we are distracted by a secondary task, said Mayhorn, who was not part of Poldrack's research team.
By relying on the habit memory system, he said, "We may find ourselves in situations where we have picked up information about performing some task but we are unsure where that information came from."
In some situations this could be dangerous, he added: "For instance, we may find ourselves making decisions based on 'gut feelings' that utilize this implicit information and not realize that our decisions may be biased by where we learned that information."
Mayhorn noted that the experiment was small, looking at 14 people from a limited age range.
"It is difficult to determine how far we can generalize these results," he said. "But I still believe that the results are interesting because they extend previous results and provide direction for future research in the area."
Poldrack's research was supported by the National Science Foundation and the Whitehall Foundation.