The brain may "tag" certain experiences for long-term storage during sleep.
Selective storage in sleep. I'm Bob Hirshon and this is Science Update.
It's long been known that sleep transfers memories to long-term storage. Now, German researchers have shown that the brain chooses which memories to store. University of Lübeck neuro-endocrinologist Jan Born and his colleagues taught volunteers either a list of paired words or a finger-tapping exercise. Afterwards, half were told there would be a test in two days. [Editor: The time they were told was actually 10 hours.] But actually, all of them were tested.
Only when they were informed about the later recall, there was a robust, very substantial benefit for these memories during sleep.
In contrast, the group that hadn't expected a test improved only a little. Born suspects that the brain tags important experiences throughout the day, and preferentially stores the tagged memories while we sleep. I'm Bob Hirshon, for AAAS, the science society.
Making Sense of the Research
We think of sleep as a time of rest, but scientists have long known that the brain is very active during sleep. We're a long way from fully understanding everything that happens during sleep, but we do know that one of its essential functions is to consolidate memories. These include declarative memories (information like names, faces, facts, and experiences) as well as procedural memories (learned skills, like playing a musical instrument or shooting a free throw). Studies show that people recall new information or perform new skills better after a night's sleep than they do after the same number of waking hours. (That's one reason pulling an "all-nighter" of study before a test isn't a great idea.)
Here, the researchers took the idea further, looking at which memories the brain actually stores. After all, we're bombarded with new information all day long, and we certainly don't remember most of it over the long term. For example, if you drive to a friend's house for the first time, you may notice the names of streets that you pass by, but a week later, you're more likely to remember the names of streets you turned on than the ones you bypassed. Which makes sense, because those are the streets you need to know in order to get to the house again. Similarly, if you think back to a history class last week, you're more likely to remember what the teacher talked about than what the person next to you was wearing. After all, you know you won't be tested on your friends' wardrobes.
In this experiment, the researchers taught volunteers one of two kinds of new memories: a list of paired words (declarative), or a finger-tapping exercise (procedural). After they taught the material, they told half of the volunteers that there would be a test 10 hours later. The reason they were told afterwards was to make sure one group didn't pay more attention during the learning phase than the other. This way, the researchers could measure the effect of just knowing the information was important.
After a night of sleep, predictably, the group that was warned about the test improved significantly in their performance and recall, compared to how they did right after the learning phase. (The volunteers were instructed not to practice the skill or think about the words in between the two sessions.) People who hadn't known a test was coming, however, improved only slightly, compared to those tested after 10 waking hours.
The results suggest that the brain doesn't treat all new memories equally. Rather, it somehow selects experiences that seem important and preferentially transfers them to long-term memory during sleep. Less-important memories may be transferred, too, but the transfer is not nearly as strong and robust. The researchers believe that the brain's prefrontal cortex—the area associated with complex thinking—"tags" the important experiences throughout the day. During sleep, the hippocampus (the brain's memory center) selectively transfers the "tagged" memories to long-term storage.
Of course, this doesn't work perfectly. We forget lots of important things (like facts for a test), and remember plenty of unimportant ones (like TV commercial jingles). But overall, our brain appears to put a higher premium on memories we're likely to need.
Now try and answer these questions:
- What role does sleep play in memory?
- What did this study add to our understanding of sleep and memory?
- Why were the volunteers not told about the test until after they learned the material, if they were told at all?
- If some volunteers had been told about the test before they learned anything, how do you think their performance would compare to the others?