Sleep Helps Us Remember What We Need To
Expectation "tags" memories for consolidation during sleep.
Posted Feb 18, 2011
The jury is back and the verdict is in. The long-term storage of memories occurs during sleep. It's not just a passive process of forgetting--in which useless or trivial short-term memories fade into obscurity. Memory storage during sleep is an active process. Several researchers have shown that neuronal representations of memories are reactivated during sleep, as if the brain were replaying a recording. The replay is essential to long-term memory storage, possibly because it redistributes neuronal connections from short-term memory to long-term storage sites in the neocortex.
The brain constructs and reorganizes its circuits while we sleep. Yet, it is apparent that there must be some selectivity involved. Most of the enormous amount of information that comes into us everyday is encoded for a short time; today we remember what we had for breakfast and where we parked the car. But as we sleep, most of that knowledge is discarded. So what we know today may be lost tomorrow--or not. Although most of what was encoded into memory is shed, the "important stuff" remains. We don't forget a disagreement with a spouse or a promising job interview.
The pivotal question is, How does the brain "decide" what to keep and what to dump? This past week, a significant study caught my eye, one that can help us eventually answer that question. German researchers have garnered evidence that the brain sorts through memories during sleep and preferentially retains the ones that are most relevant. The study, published in the Journal of Neuroscience, concludes that the brain evaluates information based on future expectations. After a good night's sleep, we remember information better when we know it will be useful in the future.
In both groups, half the volunteers were told immediately following the learning tasks that they would be tested in 10 hours. In truth, all of the participants were tested later, but only half of them expected the test. Also, some, but not all, of the volunteers were allowed to sleep between the time they learned the tasks and the time they took the tests.
Sleep compared with wakefulness produced a strong improvement on test performance only if the subjects had been informed about the test. For subjects who had not been informed, retrieval after sleep was no better than after wakefulness. Retention during the wake intervals was not affected by expectance of a test. In sum, only the people who slept and knew a test was coming had substantially improved recall. Thus, the mere expectation that a memory will prove useful in the future causes sleep to consolidate the memory.
The researchers also recorded electroencephalograms (EEGs) from the individuals who slept. Subjects who expected a test displayed a strong increase in slow oscillation activity during their slow-wave sleep. The more slow-wave activity the sleeping participants had, the better their memory during the test. Born and colleagues think that the process may involve at least two parts of the brain. The brain's prefrontal cortex appears to "tag" memories deemed potentially useful for the future, while the hippocampus consolidates those memories during sleep.
"Our results show that memory consolidation during sleep indeed involves a basic selection process that determines which of the many pieces of the day's information are sent to long-term storage," Born says. "Our findings also indicate that information relevant for future demands is selected foremost for storage."
For More Information:
I. Wilhelm, S. Diekelmann, I. Molzow, A. Ayoub, M. Mölle, and J. Born. "Sleep Selectively Enhances Memory Expected to Be of Future Relevance," Journal of Neuroscience (February 2, 2011), 31(5):1563-1569.
F. Brynie. 101 Questions About Sleep and Dreams That Kept You Awake Nights...Until Now. (Lerner, 2006).