Memory Links to 40 Winks

July 6, 2010

Science Daily/Washington University in St. Louis

When it comes to executing items on tomorrow's to-do list, it's best to think it over, then "sleep on it," say psychologists at Washington University in St. Louis.


People who sleep after processing and storing a memory carry out their intentions much better than people who try to execute their plan before getting to sleep. The researchers have shown that sleep enhances our ability to remember to do something in the future, a skill known as prospective memory.


Moreover, researchers studying the relationship between memory and sleep say that our ability to carry out our intentions is not so much a function of how firmly that intention has been embedded in our memories. Rather, the trigger that helps carry out our intentions is usually a place, situation or circumstance -- some context encountered the next day -- that sparks the recall of an intended action.


The researchers found that participants who tested in the morning following sleep overwhelmingly performed the prospective memory task better in the semantic category test, or context, than in the other two, and they found no such correlation in the group who tested sleepless.


The crux of the finding rests on the fact that the prospective memory instruction was given right after the semantic category practice. In this context, those who slept remembered the prospective memory intention better than in the other categories.


"Sleep promoted the remembering to do the prospective memory task when that one context was present, but not when some other context was present," McDaniel says. "That's because of temporal contiguity -- the fact that the participants were told to hit that 'Q' button right after they were exposed to the semantic category context.


"The idea is that the semantic category test is weakly associated with the prospective memory intention -- it's weakly floating around in the mind and becomes weakly associated with the prospective memory test," McDaniel says.


To return to the colleague and message analogy, because before sleeping you remembered you had a message to deliver to your colleague and you would see him in the conference room tomorrow, sleep enhances the likelihood that you will tell him in the conference room, but not in some other context, the office, elevator, the mail room, for example.


The researchers believe that the prospective memory process occurs during slow wave sleep -- an early pattern in the sleep cycle -- involving communication between the hippocampus and cortical regions. The hippocampus is very important in memory formation and reactivation and the cortical regions are keys to storing memories.


"We think that during slow wave sleep the hippocampus is reactivating these recently learned memories, taking them up and placing them in long-term storage regions in the brain," Scullin says. "The physiology of slow wave sleep seems very conducive to this kind of memory strengthening."

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