Sleep Affects Some Memories But Not All

It's been a long, hard drive, and you finally arrive at your friend's home so dog tired all you want to do is sleep. But your friend is eager to fill you in on the latest gossip, and yaks for four or five hours until you fade out.

When you finally wake up, you're not sure where you are, and you can't even recall the location of the nearest bathroom. But you can remember some of those gossipy tidbits with absolute clarity, especially the scary story about Uncle Ed.

Why can you remember some things so clearly, and others not at all?

Because you store different types of memories in different areas of your brain, and those various areas are very selectively effected by the timing of your sleep deprivation, according to new research at the University of Pennsylvania. At least if you're a mouse, and probably if you're human.

Timing Is Everything

The research shows that mice deprived of sleep for about five hours immediately after learning their way around a frightening new environment have lost their memory of that "context" by the time they wake up. But they won't have any trouble recognizing a sound that tells them it's time to get the heck out of there.

And that, according to assistant professor of biology Ted Abel, shows that sleep deprivation affects various memories stored in different neural systems, which are sort of like memory chips, in different parts of the brain. And the research shows that timing is everything.

"We were able to identify a particular type of memory that is susceptible to sleep deprivation," Abel says. We don't lose our ability to do everything when we need more sleep, as much of the current literature suggests, because some of our memories are more vulnerable to sleep deprivation than others, he says.

"People say sleep deprivation simply makes you unable to do everything," Abel says. "You can't hear sounds, you can't pay attention to driving or the environment. It's just a general loss of the ability to perform and respond to stimuli.

"And our experiment says that's not right, because we can have an effect in the same animal on one kind of memory and not on another," he says.

Forgetful Mice

The various memory systems in the human brain are not precisely the same as those in a mouse, but Abel believes that we, too, are probably very selective in memory retention after sleep deprivation because we also store various types of memories in different areas, or systems, of the brain.

Abel is the senior author of a report in a recent issue of Learning & Memory that discloses the results of the two-year study. His coauthors are Laurel A. Graves and Allan I. Pack of Penn's School of Medicine and Penn undergraduate Elizabeth A. Heller.

The researchers used mice as their subjects because their goal is to determine precisely which kinds of memories are stored in which areas of the brain, and how those memories are affected by sleep deprivation, and eventually the molecular mechanics that make the whole system work. It's easier to do that with mice than humans.

All the mice were taught two things — the context, or environment, in which they suddenly found themselves, and the recognition of an audible sound, or cue, that warned them of danger. Some of the mice were kept awake by gentle stroking for five hours and others were allowed to sleep for several hours.

The researchers found that the mice that were kept awake immediately after their learning session were unable to retain the memory of their environment, but had no trouble recognizing the audible cue that told them to look for the nearest exit. But the mice that were allowed to go to sleep immediately retained both types of memories.

Targeting Memory Loss

Loss of sleep more than five hours after their learning sessions had no effect on either group of mice. So the timing of the sleep deprivation was critical.

"One of the fundamental findings of modern neuropsychology is that there are specific memory systems for specific kinds of memory," Abel says. "So if you learn a motor skill, like playing the piano, or some complicated tennis stroke, that is mediated by a brain region called the stratum and the cerebellum, and if you learn a complicated thing about facts and events, that's mediated by the hippocampus. And if you learn an emotional response, to be afraid of some stimulus, that's mediated by another brain region called the amygdala."

So immediate sleep deprivation wiped out the memory in the hippocampus, but not in the amygdala, showing that lack of sleep had an impact on one type of memory, but not the other.

This type of research has implications far beyond the interest of a handful of scientists in the sleep habits of mice. Understanding how various types of memories are recorded, and the type of events or situations that can effect those specific areas of the brain, could prove invaluable in the development of drugs to help fight memory loss, and in our understanding of which memories are most vulnerable to the timing of sleep deprivation.

Lee Dye’s column appears weekly on ABCNEWS.com. A former science writer for the Los Angeles Times, he now lives in Juneau, Alaska.

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