Researchers link deep sleep phase to memory recall
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By Sandra Blakeslee
The New York Times

By isolating slabs of tissue from the brains of sleeping cats and inspecting millions of cells at a time, scientists have discovered what they think might be a key element in the brain's machinery for making long-term memories.

It seems that during an extremely quiet phase of sleep, when researchers thought that nothing much was happening in the brain, groups of cells involved in the formation of new memories signal one another. The signals, discovered only a few years ago, allow cells in many parts of the brain to form lasting links. Then, when a few of the cells are stimulated during waking hours, the links are activated and an entire memory is recalled.

The finding, described by a number of scientists at the Society for Neuroscience meeting in New Orleans last week, is part of continuing research on the role of sleep in consolidating memories. The research bears on one of the deepest mysteries of biology: Why do all animals sleep?

One idea is that sleep is critical for the maintenance and storage of long-term memories. For some time, many neuroscientists have theorized that the phase of sleep called rapid eye movement, or REM sleep, is when memories are stored. Although the new work calls that theory into question, it still suggests that we sleep, at least in part, so that we can remember, said Dr. Terence Sejnowski, a neuroscientist at the Salk Institute in La Jolla, Calif.
"Why do almost all of us need eight hours of downtime each night?" he asked. "Our sensory systems are down, our muscles are paralyzed, and we are very vulnerable. Evolution must have a purpose in mind."

During the day, he said, many bits of information enter short-term memory, but most of it is unimportant and can easily be discarded. But other information is important, Sejnowski said. So the brain needs to meld it with older memories, storing the new information as it updates older information.

The brain accomplishes this task by entering a series of different chemical and electrical states during the day and night, said Dr. Alexander Borbely, a researcher at the University of Zurich who studies sleep in people.

When people are awake, he said, their brains produce a wide variety of fast electrical activity as many neurotransmitters -- chemicals that help carry information -- are released. Cells that are involved in paying close attention to an event are especially stimulated. This heightened level of activity seems to tag them for special attention during sleep.

As people fall asleep, their brains enter a different state, Borbely said. Neurotransmitters that help keep them awake are reduced to low levels. At the same time, whole brain regions begin to oscillate or fire rhythmically at slower frequencies. People grow drowsy. During the night, different patterns of spontaneous rhythms arise in what are called sleep stages.

The REM sleep stage is what occurs when the brain becomes very active and produces dreams. Many researchers have argued that REM sleep is when memory consolidation occurs, Borbely said, but this may not be the case. Many popular antidepressant drugs essentially abolish REM sleep in people, he said, yet their memories are fine.