Evidence That Memories are Consolidated During Sleep
DURHAM, N.C. -- By exposing rats to novel objects and
measuring their brain signals, Duke University researchers have
detected telltale signal reverberations in wide areas of the
brain during sleep that reveal the process of consolidating
memories. According to the researchers, their findings offer
important evidence that extensive regions of the brain are
involved in processing memories during a particular form of
sleep, called slow-wave sleep.
The researchers said their findings lay to rest previous
doubts that sleep enables consolidation of newly acquired
memories, and also establishes roles for both slow-wave sleep
and rapid eye movement (REM) sleep in memory consolidation.
Slow-wave sleep is a deep dreamless sleep, and REM sleep is
associated with dreaming.
The researchers published their findings on Jan. 19, 2004,
in the online Public Library of Science
(www.plos.org). Senior author on the paper was Miguel
Nicolelis, Ph.D., a professor of neurobiology and of biomedical
engineering, who is also co-director of the Duke Center for
Neuroengineering. Lead author was Sidarta Ribeiro, Ph.D., in
Nicolelis's laboratory. Other co authors were neurobiologists
Damien Gervasoni, Ph.D., Ernesto Soares, Yi Zhou, Shih-Chieh
Lin, M.D., and Janaina Pantoja; and Michael Lavine, Ph.D., of
the Duke Institute of Statistics and Decision Sciences. Their
work was supported by the National Institutes of Health and the
Pew Latin American Program.
In their study, the researchers placed about 100
infinitesimal recording electrodes in the brains of rats, in
four regions involved in memory formation and sensory
processing. Those brain areas included the hippocampus, which
is widely believed to be involved in memory storage, and areas
of the forebrain involved in rodent-specific behaviors. The
scientists employed the same neural recording technology that
Nicolelis and his colleagues used to enable monkeys to control
a robot arm, an achievement announced in October 2003.
The researchers next exposed the rats to four kinds of novel
objects in the dark, since largely nocturnal rodents depend on
the sense of touch via their whiskers to investigate their
environment. The four objects were a golf ball mounted on a
spring, a fingernail brush, a stick of wood with pins attached
and a tube that dispensed cereal treats.
The researchers recorded and analyzed brain signals from the
rats before, during and after their exploration, for several
days across natural sleep-wake cycles. Analyses of those
signals revealed "reverberations" of distinctive brain wave
patterns across all the areas being monitored for up to 48
hours after the novel experience.
According to Ribeiro, "We found that the activity of the
brain when the animal is in a familiar environment does not
'stick' -- that is, the brain keeps moving from one state to
another. In contrast, when the animal is exploring a novel
environment, that novelty imposes a certain pattern of
activity, which lingers in all the areas we studied. Also, we
found that this pattern was much more prevalent in slow-wave
sleep than in REM sleep."
Conversely, previous studies by Ribeiro and his colleagues
demonstrated that the activation of genes able to effect memory
consolidation occurs during REM sleep, not slow-wave sleep.
"Based on all these results, we're proposing that the two
stages play separate and complementary roles in memory
consolidation," he said. "Periods of slow-wave sleep are very
long and produce a recall and probably amplification of memory
traces. Ensuing episodes of REM sleep, which are very short,
trigger the expression of genes to store what was processed
during slow-wave sleep." In principle, this model explains
studies such as those by Robert Stickgold and his colleagues at
Harvard University, showing that both slow-wave and REM sleep
have beneficial effects on memory consolidation, he said.
According to Nicolelis, the new experiments remedy shortcomings
of previous studies.
"I think that this is another demonstration of the power of
the capability of looking at multiple areas of the brain
simultaneously," he said. "Previously, investigators have
reported the possibility that memories are consolidated during
sleep by looking at reverberations, but they only looked in the
hippocampus and cerebral cortex. And they only looked for an
hour or so. They never looked at several regions of the brain
simultaneously, and they never looked for longer periods of
time. We've now demonstrated that these reverberations occur in
a much more distributed manner over the forebrain, and for a
very long time period. Importantly, emphasized Nicolelis, the
latest findings provide further evidence that the brain behaves
as an integrated whole in processing information.
"The brain cannot be seen as just a mosaic of structures,
with one performing a particular function and others doing
other unrelated functions," he said. "This model has to be
discarded, and this paper is one of the first studies to show
that the brain has to be considered as a whole. So, while
different aspects of memory consolidation may be happening in
different structures, the whole brain is participating in this
process, and not just the hippocampus or cortex, which was the
idea prior to this work," said Nicolelis.
Next, said Nicolelis, the researchers will perform
experiments in which they record from more brain structures
over longer time periods. They will also genetically manipulate
the animals, switching off specific genes to attempt to affect
neural circuitry involved in memory storage.