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Why it's hard to remember two new things

Studying for multiple final exams was always rough. Just when you felt you’d mastered Spanish, say, and moved on to American history, every bit of grammar you’d slaved to store away would just stream out of your head. It seemed like your brain wasn’t big enough to keep both subjects going at the same time.

As it turns out, you were right -- sort of. Scientists have now discovered why it’s really hard to learn two subjects, one right after the other. A new study published in Nature Neuroscience shows that when you try to learn or to memorize two different types of information in rapid succession, the second interferes with the brain’s ability to permanently store the first.

To prove this was happening, Dr. Edwin Robertson and his colleagues at Harvard rounded up 120 college-age students for an experiment in which study volunteers were given two memory tasks back to back.

First the volunteers were given a list of words to memorize. Then they were given a finger-tapping task -- unbeknownst to the volunteers, there was a pattern to the finger tapping that they could unconsciously learn through repetition.

Right after the word test, volunteers remembered the list quite well. But after they did the finger tapping routine, they’d forgotten many of the words. 

Then, in the second part of the experiment, the tasks were reversed so finger tapping came first, followed by the word list. Once again, the volunteers did well in testing after the first task, but after performing the second, they’d lost much of what they learned in the first.

That could just mean that the brain couldn’t hold all that information, says Robertson, an associate professor of neurology at Harvard Medical School and Beth Israel Deaconess Medical Center.

So Robertson and his colleagues re-ran the experiment, only this time they used a magnetic device that pumps up the electricity in the brain when the device is placed against the head.

Sure enough, with the stimulation people remembered both tasks as well as if they’d done each one alone. What’s interesting is that the stimulation didn’t improve memories when only one task was done at a time. So it was just removing interference, says Robertson.  

Practically speaking, what the study is telling us is that when you’re trying to learn two different things, you need to take a break in between. Other research shows that two hours might be enough, Robertson says. 

There might be other ways to trick the brain into giving up interference, but that might not be a good strategy.

It’s not clear why the brain is wired the way it is, says Robertson. “An important aspect of the study is that it demonstrates that the brain actively conspires to produce memory interference and so impair recall,” he adds. “Admittedly this may seem somewhat paradoxical, and one way to resolve that paradox is to imagine that memory interference serves some important function. As yet, we don't know that that function may be.”

And that’s why we probably shouldn’t be trying to fool Mother Nature by getting around the interference issue, Robertson says.