From: Brian Atkins (brian@posthuman.com)
Date: Fri Sep 19 2003 - 12:07:29 MDT
We're going off topic, but there was a nice feature in New Scientist on
this recently:
To sleep, perchance to dream
New Scientist vol 178 issue 2401 - 28 June 2003, page 28
(plus two sidebar articles)
--------------------------------------------------------
In the year that Everest was conquered and DNA's structure discovered, a
young PhD student called Eugene Aserinsky did what no one had thought to
do before. He spent hours staring at the eyelids of a sleeping person.
What he saw was amazing. Instead of the slow periodic motion he'd been
expecting, he saw frantic, jerky eye movements.
Until then, sleep had been seen as a pretty passive and uninteresting
state of mind. But Aserinsky and his PhD adviser, physiologist Nathaniel
Kleitman of the University of Chicago, went on to show that this "rapid
eye movement" was correlated with massively increased brain activity and
dreaming. Their discovery is widely regarded as having kick-started the
modern discipline of sleep research.
Fifty years of intense study later, however, and almost every aspect of
sleep remains a mystery. We still don't know what sleep is for, how it
evolved, or why we dream. But there are exciting new ideas.
Earlier this month, the city of Chicago hosted the biggest ever
gathering of sleep researchers to mark the 50th anniversary of the
discovery of REM, and to ask again, what is it all for? Graham Lawton
reports.
MOST PEOPLE think they don't get enough of it. Some manage just fine on
half the usual amount. Dolphins do it with half a brain at a time. Rats
die after three weeks without it, yet male Emperor penguins manage an
entire three months of brooding without getting any. It's as good as
ubiquitous among animals - even insects do it. We spend a third of our
lives at it. Yet no one has any clue why we do it or how it came about.
Our ignorance about sleep is becoming something of an embarrassment. Try
to think of another fundamental biological phenomenon to which we can't
assign a role. You'll draw a blank. "I think it's the biggest unanswered
question in neuroscience," says Craig Heller, a sleep researcher at
Stanford University in California.
Our ignorance does not stem from lack of trying. There are dozens of
theories of sleep function out there, which fall into four broad
classes: restoration and recovery, predator avoidance, energy
conservation and information processing. Most sleep researchers are
happy to accept that sleep has more than one function and that several
of these theories, or perhaps all of them, are simultaneously correct.
And yet not one of them has been confirmed or refuted. Take "restoration
and recovery", for example. This is probably the most obvious potential
function of sleep and surely the easiest to test. Yet we still don't
know what the brain might be restoring, or how.
Why is sleep such a hard nut to crack? One big stumbling block has been
its dual nature. While we sleep, we cycle between two very different
states (see Diagram). The first is called slow wave sleep because it is
characterised by long, lazy waves of undulating electrical activity
(delta waves) that seem to be synchronised across the whole brain. The
second is rapid eye movement (REM) sleep, as Aserinsky first witnessed,
and it could hardly be more different. REM sleep is characterised by
frantic brain activity that looks very much like wakefulness on an EEG
trace. It also has very obvious physical signs: the rapid flickering
motion of the eyeballs, the near-total muscle paralysis thought to
prevent you acting out your dreams, and penile erections (women exhibit
engorgement of the clitoris and lubrication of the vagina).
REM sleep appears to have arisen quite early in evolution - reptiles,
birds and mammals all do it. It's also a very active and excitable
state. So it must serve a very useful function. Any decent theory of
what sleep is for must account for it. In fact, explaining REM has so
far dominated sleep research (so much so that slow wave sleep, despite
having several distinct stages or depths is often defined merely as
non-REM sleep). Yet the function of REM has proved so difficult to
fathom that some sleep researchers despair that we will ever solve the
problem. "REM sleep has a biological function but we have not been able
to find it. And I don't think there will be any explanation for another
50 years," says Michel Jouvet, a veteran sleep researcher from the
medical school at the University of Lyon in France, who has spent 48
years studying REM sleep.
While plenty of researchers are still busy studying REM and dreaming
(see Box, "What dreams are made of"), in the past few years a new
generation of sleep function theories have emerged that relegate REM to
the background and say that non-REM is what sleep is all about. In these
theories, REM sleep is a mere handmaiden of non-REM sleep and its
function is to give the brain a break from, or perhaps even test and
modify, the really crucial activity that is going on during non-REM
sleep. Can this new generation of ideas finally crack the problem?
One such theory has been developed by Heller. Some six years ago, he
noticed something odd about the electrical activity of the brain during
non-REM sleep. Looking closely at an EEG, he saw brief and subtle
patterns of activity that looked like aborted transitions into REM. Such
episodes, he found, happen increasingly often until the brain eventually
swaps entirely into REM sleep. In other words, the pressure to enter REM
sleep seems to build up through a bout of non-REM until the brain can no
longer hold it back.
This, Heller says, is a potentially revolutionary observation. Most
theories of sleep control posit an external master switch that flips the
sleeping brain from one state to another on a preordained timetable (the
classic 90-minute cycle). Both states of sleep then have plenty of time
to get on with their respective functions. But the build-up of aborted
transitions to REM suggests that something else controls the change.
It's as if non-REM creates the need for REM sleep - just as wakefulness
eventually creates pressure to snooze. Further support for this idea
comes from the fact that the length of a bout of REM is directly
proportional to the preceding bout of non-REM. The longer you spend in
non-REM, the more REM you need to "recover", perhaps.
In this view of sleep, REM is demoted to a secondary role as a recovery
phase after whatever hard work is done during non-REM. "Our premise is
that something happens during non-REM - some imbalance builds up - that
has to be corrected for in REM," says Heller.
But what goes on in non-REM that means you need to recover afterwards?
According to Heller, it's all to do with energy replenishment. During
non-REM sleep your brain tops up its turbochargers - glycogen stores in
the brain's supporting glial cells that neurons draw on when they
absolutely have to work at maximum capacity. When they run down, they
must be replenished.
To do this, the brain would have to enter a quiescent state. To achieve
this, suggests Heller, it might throw its neuronal membranes into a
suppressed electrical state called "hyperpolarisation", making them
inactive and unresponsive to outside stimuli. This would manifest itself
as the unconsciousness of slow wave sleep. Hyperpolarisation, however,
comes at a price. It can only be achieved by allowing precious potassium
ions to leak out of neurons. Such a state could only be maintained for
so long without risking permanent loss of ions. Every now and again the
brain has to pump potassium ions back in, repolarising the membrane and
switching the cortex back on again, all the while keeping you in a
relatively quiescent state. This is what we experience as REM sleep. All
the exciting and spectacular facets of REM sleep are merely a by-product
of the brain's housekeeping, no more and no less, says Heller. "Dreaming
and so on are really a second or third order phenomenon and are not a
function of REM sleep at all."
But isn't this rather disappointing? Can all the wonderful complexities
and experiences we have in REM sleep really come down to brain cells
hauling potassium ions across their membranes? Heller chuckles and
points out the hypothesis remains largely untested; there is plenty of
room for surprises. "I'm sure it's going to turn out to be more complex
than that."
Meanwhile, other ideas are emerging that put non-REM in the spotlight
without relegating REM sleep completely to the level of the mundane. One
hypothesis has been developed by Terrence Sejnowski of the computational
neurobiology laboratory at the Salk Institute in La Jolla, California.
In his hypothesis, non-REM sleep is also used for recovery and
restoration, carrying out "construction projects" related to the rigours
and events of its waking activity - replenishing proteins, strengthening
synapses, inserting receptors into membranes and so on. You need to be
unconscious during this activity so that there is no neuronal activity
to get in the way - a bit like moving out of your house while the
builders are in. Then, once the brain has completed some construction
work, it flips into REM for an interim report. In effect the brain boots
up some of its waking systems in a contained environment to test what it
has done and find out what still needs to be completed. Then it slips
back into non-REM sleep and gets on with the construction work.
"As you have experiences during the day, you're making a lot of small,
temporary changes here and there," says Sejnowski. "You need to organise
and prioritise this information, and you do it in slow wave sleep." The
evidence he has for this assertion comes from computer models of a
phenomenon called "spindling" - spikes of electrical activity seen only
during the transition from wakefulness to non-REM, and from REM back
into non-REM. During spindling there is massive influx of calcium into
neurons that have been modified by that day's experience. And calcium is
known to be a crucial activator of enzyme function and gene expression -
the kinds of biochemical heavy-lifting the brain would have to perform
to convert these small changes into permanent ones.
"But you don't want to make all your changes at once," Sejnowski says.
"You make a few of them, and you test to see what impact this has. Maybe
REM sleep is the test period."
There is other evidence building for the idea that a brain's workload
determines how much deep sleep a brain needs, perhaps even with
harder-working areas showing different sleep states to those that had a
more leisurely day (see "Sleep or sleeps?").
One of the beauties of these "non-REM rules" ideas is that they help
explain why the brain cycles through several non-REM to REM bouts during
a night's sleep, and why you always start the night with a bout of
non-REM. If the two different modes of sleep have independent functions,
then it's not clear why they should be organised in this way. But if
non-REM sleep makes REM essential, then the way sleep is organised
suddenly makes much more sense.
Another leading researcher who sees REM as having a secondary but
non-trivial function is Derk-Jan Dijk, director of the newly opened
Sleep Research Centre at the University of Surrey in Guildford. He's
sure that the main function of sleep is restorative and that the
restoration process - whatever it is - has little to do with REM sleep.
He points out that bouts of REM increase in length throughout the night
and hit a peak at the very end of the sleep. "This tells you something
about the function of REM sleep," he says. "It may not be that important
to the recovery process."
So what is it for? "It may be more important for the transition from
sleep and wakefulness. Some people have said that REM sleep is a gate to
wakefulness." Animals very frequently wake up at the end of each REM
episode, check all is well, then drift off again. Even humans may do it
when there is something, say an infant, in need of regular checking.
Another new theory pushes the importance of REM sleep even further down
the list, at least in adults. According to this idea, REM does have a
function - but only in the womb and soon after birth. REM sleep
continues in adults only as a developmental relic of this function. It's
the brain's belly button.
The idea originated from a group led by Stephen Duntley of the Sleep
Medicine Centre at Washington University in St Louis, Missouri. They
started from the well-known fact that as the brain develops, it prunes
out redundant neurons in the cortex to build a sleek and honed thinking
machine. The pruning tool is programmed cell death, or apoptosis, and
the signal for it to occur is neuronal inactivity. Duntley's group
believes the brain needs quiet periods of recovery and restoration, but
this creates a problem in babies. You don't want to lose too many
neurons as they descend into their quiescent sleeping state.
The group also noted another well-known fact - that infants have a very
high proportion of REM sleep, starting in the womb and continuing
throughout early childhood. During REM sleep, neurons are very, very
active indeed. From an EEG trace alone you would have a hard time
distinguishing between REM and wakefulness. Could the two phenomena be
related? Is frantic brain activity in REM anything to do with building a
brain?
To find out, group member Michael Morrissey used a drug called clonidine
to selectively deprive very young rats of 60 per cent of their normal
REM sleep. When they examined the rats' brains for markers of cell
death, the results were clear: REM-deprived baby rats had huge amounts
of apoptosis, much more than you would see in a healthy brain or in an
adult given the same drug. "It's way beyond normal," says Morrissey.
The team speculates that REM sleep evolved to keep useful neurons busy
and so save them from apoptosis, without forcing the animal to waste
precious energy on staying awake. "It's a very exciting theory," says
Duntley.
Critics agree that it's an interesting idea, but are not yet convinced
by the data. David Gozal, an expert in childhood sleep at the University
of Louisville in Kentucky, points out that the apoptosis assay the team
used is not very sensitive. Given that there is so much apoptosis going
on at this stage of development, it's hard to say for sure that there's
an abnormal load in REM-deprived rats. Duntley counters by saying that
the work is at a very early stage and the hypothesis should be easily
testable with refined techniques. Watch this space.
But there is another prominent group of theorists who refuse to accept
that sleep is merely a time for rest and recuperation. They prefer to
assign sleep - particularly REM - a more fittingly cerebral role in
information processing, categorising and storing memories, perhaps, or
even thinking creatively.
The idea that sleep plays a crucial role in learning and memory goes
back to 1983 at least, when James Watson of double-helix fame, proposed
that REM was responsible for the destruction of unwanted facts and
experiences. Since then the field has exploded, producing reams of
evidence that REM sleep is intimately involved with what's known as
"procedural memory" - learning how to perform a complex task such as
riding a bike or playing the piano. (The other sort of memory,
"declarative memory" for storing facts and figures, seems to be
unaffected by sleep.)
In one classic experiment, volunteers were asked to spend four hours
playing an apparently simple video game involving six targets with
buttons underneath. The rules are very straightforward: when a target
lights up, subjects press the corresponding button. The researchers
measure their reaction times to see how good they are at the game. The
more subjects play, they better they get. What the players don't know,
however, is that the targets don't light up at random. There is a
complex imperceptible set of rules determining the sequence with which
they light up.
Then comes the interesting bit. The volunteers are allowed a good
night's sleep and then retested. Miraculously, their performance
improves a great deal overnight. But not in every case. In some
volunteers there is no improvement at all. The difference? One set of
non-improvers was deprived of REM sleep by repeatedly being wakened
through the night. The other was playing a game in which the sequence
was totally random. There's nothing to learn, and so no overnight
improvement. The conclusion? Spending time in REM asleep helps
"consolidate" procedural memory.
Many similar experiments have come to the same conclusions about the
role of REM sleep in procedural memory. And there is supporting evidence
from other types of experiment, too. For example, PET imaging of the
brain as it learns a procedural task shows a characteristic pattern of
activity which is repeated again and again during REM sleep. This
activity is taken to be memory consolidation in action.
But once again, it is a measure of non-REM's growing influence and of
REM's waning one that researchers have begun to grant it a central role
in information processing too (New Scientist, 25 September 1999, p 26).
This is quite a reversal. "REM sleep was seen as the most important, but
now slow wave sleep is moving up fast," says Carlyle Smith, the director
of Trent University's sleep labs in Peterborough, Ontario, and a leading
proponent of sleep's role in memory consolidation.
In particular it seems that slow wave sleep is when the brain processes
spatial memories - learning your way around a new city, for example. In
one experiment, volunteers had to learn to navigate their way around a
virtual city using a joystick while their brains were being imaged with
PET. As they learned the routes, the PET scan showed intensive activity
in the hippocampus, as expected from previous work showing its vital
role in spatial learning.
The subjects then slept while hooked up to the PET machine. During slow
wave sleep their hippocampuses showed exactly the same patterns of brain
activity as during learning, and the next morning they were much better
at finding their way around. "Slow wave sleep really has something to do
with memory," concludes Philippe Peigneux of the University of Liège in
Belgium, who carried out the tests.
The idea that you learn in your sleep has obvious appeal and has lodged
in the popular consciousness. But not everyone thinks it is correct.
There is a small and highly vocal group of critics who say that sleep
has absolutely nothing to do with memory. It looks as though even this
most compelling of sleep function theories will have to stay on the
"unproven" pile until more evidence comes in.
One of the most strident critics is Robert Vertes of the Center for
Complex Systems and Brain Sciences at Florida Atlantic University, Boca
Raton. He claims that the evidence for a role for REM sleep in memory is
highly contradictory, with about half the published studies showing
sleep has no effect. And in those that show a positive effect you cannot
control for the stressful effects of REM sleep deprivation. Perhaps
sleep-deprived subjects perform worse simply because they are tired.
Vertes's most compelling evidence, however, comes from patients with
brain injuries who do not get any REM sleep at all. In one remarkable
case, a man was hit by shrapnel from a bullet at the age of 20 which
deprived him almost completely of the ability to enter REM sleep. At
best he was getting 1 to 2 per cent REM sleep at night. Yet he continued
his education and became a practising lawyer. It's hard to imagine
someone severely deficient in procedural memory achieving this, Vertes says.
Another line of counter-evidence comes from animals. "If you think about
the theories of REM sleep serving some intellectual function, the fact
that most mammals have about the same percentage of REM sleep gives you
pause for thought," says Heller. "Does a cow really have as many
problems as we do?" A similar point is made by Jerome Siegal of the
University of California, Los Angeles. He points out that there seems to
be no correlation between mental capacity and REM sleep duration (see
Diagram). Platypuses, for example, sleep for about 14 hours a day, of
which 8 hours are REM sleep. "The platypus is a lovely animal, but it's
also a rather stupid animal," says Siegal.
The theory's proponents won't have any of it. Robert Stickgold of
Harvard Medical School, for example, says that evidence in favour of
information processing is so good that it cannot be disputed. The best
studies control for tiredness and have confidence limits of 0.001, he
says. And the animal comparisons are meaningless and unscientific -
rather like saying that because millipedes have dozens of legs yet move
more slowly than cheetahs, legs cannot be for locomotion.
So, half a century on from the start of modern sleep science, are we any
closer to understanding what sleep is for? Things have at least moved on
from the days when sleep was simply a form of inactivity and dreams were
for prying into your private desires, and might well have come from the
kidneys, for all the biology that was known. And there are plenty of new
techniques coming into their own that might help settle some of the
arguments. The ability to make tissue slices "sleep" in a dish, better
and better brain scanners, new mass genetic screening techniques, mutant
flies with sleep deficits, and connections with the fields of circadian
rhythms and metabolic control - any one of these new developments could
be the key to the puzzle. The one thing everyone agrees on is that it is
a goal worth pursuing. "If we did know what the function of sleep was,
things would change dramatically," says Heller.
---------------------------------------
What dreams are made of
Move over Freud. Modern sleep researchers are starting to analyse dream
content, and it has nothing to do with your mother
THE 1953 discovery of REM sleep and its clear link with dreaming
(Science, vol 118, p 273) gave a huge boost to dream researchers. Until
this point, the study of dreams was almost the exclusive domain of the
Freudians, who believed the content of our dreams was a hotline to our
innermost desires and feelings. But the obvious mental and physical
signatures of REM sleep - rapid eye movements, frantic brain activity,
and (to the delight of the Freudians) penile erections - opened the door
to studying dreams as a real biological phenomenon. "We rejoiced in the
discovery of REM sleep," recalls J. Allan Hobson, a veteran psychiatrist
and dream researcher at Harvard Medical School. "We thought it would
make psychoanalysis scientific."
It wasn't as easy as they hoped. Researchers failed to find any
consistent links between physical and mental activity and dream content.
Eye movements, for example, rarely tracked the visual content of dreams.
Erections had little to do with dreams' erotic content. Worst of all,
within a decade, the apparently clear-cut connection between REM sleep
and dreaming melted away, as researchers noted almost as many dream
reports when people were woken from non-REM sleep as from REM. If
dreaming could come from two such disparate brain states, how would we
ever explain it?
In the past few years, however, the biological study of dreaming has
undergone something of a revival. Cognitive neuroscientists and
neurologists are getting in on the act, using new tools to work out what
causes the experience of dreaming and even what its function might be.
One of the key discoveries is that dreams reported after waking from
non-REM sleep may originate in REM sleep after all. Using sensitive
electrical monitoring techniques to eavesdrop on the activity of the
sleeping brain, Tore Nielsen of the University of Montreal and others
have spotted brief fragments of REM, so called "covert REM", intruding
into non-REM sleep. These were just too subtle to be picked up by
standard equipment. Another recent study led by Hiroyuki Suzuki of
Japan's National Institute of Mental Health in Ichikawa has confirmed
that, despite prodigious dream reports from non-REM awakenings, there is
a huge qualitative difference between the two kinds of dreaming. Non-REM
dreams tend to be short and dull. REM dreams, in contrast, are vivid and
long.
The re-corralling of the richest dreams back into REM sleep, when brain
activity is at its liveliest, has encouraged other researchers to take a
fresh look at dreams to see what they tell us about cognition during
sleep. "Dreams are the only source of cognitive information we've got,"
says Sophie Schwartz, from the University of Geneva in Switzerland.
One big question is whether the brain activity you see in REM sleep
corresponds with the experiences we call dreams. To find out, a team
lead by Pierre Maquet at the University of Liège in Belgium made a
"sleep map" of the brain, using the imaging technique PET to find out
which areas were busiest during the different phases of sleep. Their
results showed that brain activity in REM is very different from non-REM
or wakefulness - and that it tallies nicely with the content of dreams.
During dreams, visual areas are very active, as are the amygdala,
thalamus and the brainstem, which fits with the fact that dreams tend to
be very visual and emotional. At the same time, the prefrontal and
parietal cortices and the posterior cingulate, areas which deal with
rational thought and attention, are all very quiet, which tallies with
the lack of insight, illogicality and time distortion that characterises
dreams (see Diagram).
Maquet accepts that this conclusion is too general to give any major
insights into the process of dreaming. But there is another promising
approach on the horizon. Schwartz, who was once part of Maquet's group,
suggests we can learn from patients with neurological damage for whom
everyday experience is full of bizarre events most of us only experience
while dreaming.
Schwartz is currently focusing on Frégoli syndrome, in which people
constantly mistake strangers for people they know, even though there are
no physical similarities. It's a mistake that happens all the time in
dreams. Frégoli syndrome is caused by brain damage which severs the link
between the prefrontal cortex and the other brain structures involved in
recognising faces. The way she sees it is this: if you monitor these
areas in normal sleeping subjects, you can ask whether Frégoli-type
dreams coincide with Frégoli-type brain activity.
There are numerous other brain lesions that cause dreamlike perception
during waking - reduplicative paramnesia, in which patients recognise
new places as familiar, micropsia and macropsia, where you see things as
too big or too small, palinopsia, where you see multiple copies of an
object, and achromatopsia, in which colour perception goes haywire.
Schwartz says that all of these could help us make links between brain
activity and standard dream events.
But none of this answers the most critical and intriguing question: what
are dreams for?
Are we any closer to an answer?
"I don't think we know anything with any confidence about what dreams
are for," says psychiatrist Robert Stickgold of Harvard Medical School.
But he has some ideas. Stickgold believes that one of the critical
functions of dreaming is information processing, including memory
consolidation (see main story). But he's recently extended the idea into
a wider concept of information processing he calls "finding meaning".
Sometimes when you have a difficult decision to make, all the rational
thinking in the world won't give you the answer, he points out. So what
do you do? "You go home that night and you sleep on it," says Stickgold.
By the morning you somehow have the answer even though you've gained no
new information overnight.
It is dreaming during REM sleep, he thinks, that performs this magical
decision-making process. As you dream, your brain runs through imaginary
scenarios, testing your emotional response to them without rationality
getting in the way. He now has experimental evidence that REM sleep
promotes creative thought, allowing you to bring together widely
differing concepts you would never link while awake.
The evidence comes from a common cognitive test of a process called
priming. Normally the investigator would show someone a word, then
quickly flash up another, either real or meaningless. The task is to
work out whether it's a real word or not. If the second word is related
to the first, say the pair were "wrong" and "right", then the decision
time is usually significantly faster than, say, "wrong" and "paper". The
reason, according to conventional theory, is that the first word has
primed the brain to recognise related words by activating networks of
associated concepts.
Stickgold, though, was not interested in wakeful consciousness. He
tested people who had just been roused from REM sleep. The result was
the exact opposite of wakefulness. The more distantly related the second
word, the faster the subjects recognised it. "In REM sleep, the brain
ignores the obvious in favour of the crazy, the unexpected or the
bizarre," Stickgold says. "It's biased towards activating weak,
non-obvious and potentially useful connections." And this, he says, is
what allows us to make meaning out of complex information. It might even
be the origin of creativity.
-----------------------------------
Sleep or sleeps?
Do different parts of your brain sleep in different ways? Ever since the
discovery of REM we have known that there are several distinct states of
sleep. Now it looks as though different patches of the brain can be in
different sleep states at the same time. "Sleep is not a whole-brain
phenomenon," says James Krueger at Washington State University in
Pullman. "It's a localised property."
Dolphins and some birds and fish are already known to sleep with one
cerebral hemisphere at a time, either to stay alert or so they don't
drown. But Krueger's idea runs deeper, suggesting that sleep is a
completely devolved system, with different brain regions making their
own decisions about depth and onset of sleep. His concept of sleep is at
an early stage, and he has just started revealing his evidence to other
researchers. But if he is right, then it could give us some new hints
about what sleep is for.
The first sign that sleep states might differ between brain regions came
from studies of "sleep regulatory substances", biochemicals that build
up in the brain during wakefulness and apparently help trigger the
transition into sleep. Around 10 years ago Krueger noticed that these
substances build up faster in parts of the brain that are most active
during wakefulness. Did this have any effect on subsequent sleep?
It turns out that it does. Apply extra amounts of sleep substances to
one hemisphere of the cortex and not the other and the result is much
deeper non-REM sleep on that side, as well as subtle differences in REM
activity. It seems as if the harder a brain region works during the day,
the harder it has to sleep at night.
This tallies with other findings. Cats and rats that are kept in the
dark during wakeful hours have unusually shallow non-REM sleep in the
visual cortex, but much deeper non-REM sleep in the part of the cortex
dealing with touch. And if you trim a rat's whiskers on one side then
put it into a maze, the corresponding half of its brain sleeps more
deeply later on - presumably because it had to work harder processing
incoming data.
And according to David Rector of Washington State University, you can
even see devolved sleep in an EEG trace. He says that a close reading of
EEGs reveals that what looks like a steady state is anything but. Though
the overall effect might look uniform, on a more local level brain
activity is highly changeable, with different neuronal groups constantly
cycling between states. What's more, Krueger points out that in people
who survive brain injury, regardless of which parts of the brain are
lost the patient always remains capable of sleep. This suggests that
sleep is an intrinsic property of brain cells and doesn't need to be
imposed from the top.
Support for this view is also coming from experiments on groups of
neurons grown in a Petri dish, which spontaneously produce states very
like non-REM sleep. Deprive these neurons of their shut-eye and they go
haywire, firing rapidly and randomly in an epileptic-like state. Sleep,
then, seems to be an inherent property of small neural networks which
keeps them in good shape. Is this how sleep evolved and why we have to
do it?
Perhaps the most intriguing spin-off from this work is to understand the
mysterious "state dissociation" disorders - cataplexy, for example,
where the loss of muscle tone designed to stop you from acting out
dreams unexpectedly switches on during wakefulness and makes you fall
over, or dream enactment where muscle tone remains switched on in REM
sleep. Doctors have long assumed these are caused by some unspecified
mixing of states of consciousness. Krueger's theory might just prove
them right.
-- Brian Atkins Singularity Institute for Artificial Intelligence http://www.intelligence.org/
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