SLEEP TWITCHES LIGHT UP THE BRAIN
University of Iowa
study has found twitches made during sleep activate the brains of mammals
differently than movements made while awake.
Researchers say the
findings show twitches during rapid eye movement (REM) sleep comprise a
different class of movement and provide further evidence that sleep twitches
activate circuits throughout the developing brain. In this way, twitches teach
newborns about their limbs and what they can do with them.
"Every time we
move while awake, there is a mechanism in our brain that allows us to
understand that it is we who made the movement," says Alexandre Tiriac, a
fifth-year graduate student in psychology at the UI and first author of the
study, which appeared this month in the journal Current
Biology. "But twitches seem to be different in that the brain
is unaware that they are self-generated. And this difference between sleep and
wake movements may be critical for how twitches, which are most frequent in
early infancy, contribute to brain development."
Mark Blumberg, a
psychology professor at the UI and senior author of the study, says this latest
discovery is further evidence that sleep twitches -- whether in dogs, cats or
humans -- are connected to brain development, not dreams.
"Because
twitches are so different from wake movements," he says, "these data
put another nail in the coffin of the 'chasing rabbits' interpretation of
twitches."
For this study,
Blumberg, Tiriac and fellow graduate student Carlos Del Rio-Bermudez studied
the brain activity of unanesthetized rats between 8 and 10 days of age. They
measured the brain activity while the animals were awake and moving and again
while the rats were in REM sleep and twitching.
What they discovered
was puzzling, at first.
"We noticed
there was a lot of brain activity during sleep movements but not when these
animals were awake and moving," Tiriac says.
The researchers
theorized that sensations coming back from twitching limbs during REM sleep
were being processed differently in the brain than awake movements because they
lacked what is known as "corollary discharge."
First introduced by
researchers in 1950, corollary discharge is a split-second message sent to the
brain that allows animals -- including rats, crickets, humans and more -- to
recognize and filter out sensations generated from their own actions. This
filtering of sensations is what allows animals to distinguish between
sensations arising from their own movements and those from stimuli in the
outside world.
So, when the UI
researchers noticed an increase in brain activity while the newborn rats were
twitching during REM sleep but not when the animals were awake and moving, they
conducted several follow-up experiments to determine whether sleep twitching is
a unique self-generated movement that is processed as if it lacks corollary
discharge.
The experiments were
consistent in supporting the idea that sensations arising from twitches are not
filtered: And without the filtering provided by corollary discharge, the
sensations generated by twitching limbs are free to activate the brain and
teach the newborn brain about the structure and function of the limbs.
"If twitches
were like wake movements, the signals arising from twitching limbs would be
filtered out," Blumberg says. "That they are not filtered out
suggests again that twitches are special -- perhaps special because they are
needed to activate developing brain circuits."
The UI researchers
were initially surprised to find the filtering system functioning so early in
development.
"But what surprised
us even more," Blumberg says, "was that corollary discharge appears
to be suspended during sleep in association with twitching, a possibility that
-- to our knowledge -- has never before been entertained."
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