By Will Dunham
WASHINGTON
(Reuters) - Sleep, whose need is not questioned but whose function
is shrouded in mystery, performs a critical role in brain development
in early life, scientists said on Wednesday.
Researchers
at the University of California at San Francisco found sleep dramatically
enhances changes in brain connections during a vital period of
visual development in young cats.
They examined
the effect of sleep on the brain after cats experienced an environmental
challenge -- vision blocked in one eye for six hours. They found
that cats allowed to sleep for six hours after the experience
developed twice the amount of brain change as those kept awake
during that period.
The study
appeared in the journal Neuron.
UCSF researcher
Marcos Frank noted that every animal sleeps -- even flies experience
a sleep-like state. But despite major progress in the understanding
of the neurobiology of sleep and the consequences of sleep deprivation,
Frank said the reason the brain needs sleep has remained a mystery.
Michael Stryker,
who heads the university's department of physiology, said the
study may shed some light on the matter.
``One of the
aspects of the mystery is that young animals and human babies
sleep a whole lot more than they do when they get older,'' Stryker
said in an interview. ``And this is precisely the time in life
when the connections in the brain are being reorganized to attain
the perfect precision that they have in normal adults.''
The capacity
for brain change -- growth and strengthening of connections between
nerve cells -- is the basis of early development in the brain,
the researchers said. The enhancement and refinement of neural
circuitry continues to a lesser extent in the adult brain, they
added.
The process
of growth, known as plasticity, is believed to underlie the brain's
capacity to control behavior, including learning and memory. Plasticity
occurs when neurons are stimulated by events, or information,
from the environment.
Strong Evidence
On Function Of Sleep
The study
represented strong evidence that one function of sleep is to help
consolidate the effects of waking experience on cortical plasticity,
putting memories into permanent storage, Stryker said. While the
study examined sleep's effect on young cats, sleep may play a
similar role in older animals and people, he added.
``There may
be similar phenomena going on in other areas of the brain later
in life,'' Stryker said. ``I think it's likely to be true that
other areas of the brain, higher areas of the brain, have their
critical (developmental) periods later in life -- and some of
them, in the highest areas, the critical periods never close until
senility.''
The study
found the amount of plasticity in the brain depended on the amount
of sleep known as non-rapid eye movement -- the deep, quiet sleep
marked by large, slow brain waves. This type of sleep alternates
with periods of rapid eye movement, or so-called ``dream'' sleep,
which is marked by rapidly changing brain waves and bursts of
eye movement.
The researchers
measured in cats the response of neurons of the brain's visual
cortex to the six hours of blocked vision. They said the visual
deprivation triggered a rapid remodeling of neural circuitry known
as ocular dominance plasticity.
The researchers
then examined the impact of sleep on the long-term effects of
those changes by using brain imaging and making electrical recordings
from brain cells.
The researchers
determined that animals allowed to sleep for six hours after the
period of visual deprivation developed twice the amount of brain
change as those cats kept awake in a dark room during those six
hours. The cats permitted to sleep also experienced twice the
amount of change as those evaluated immediately following the
period of visual deprivation.
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