OT. brain article, default mode network

http://www.sciencenews.org/view/feature/id/45178/title/You_Are_Who_You_Are_by_Default

Home / July 18th, 2009; Vol.176 #2 / Feature You Are Who You Are by
Default
It may be off when you’re on, but the brain network behind daydreams
and a sense of self is no slackerBy Tina Hesman Saey July 18th, 2009;
Vol.176 #2 (p. 16)


Wandering and wondering The brain's default mode network -- a series
of connected areas that work hardest when most of the brain is at rest
-- is active during daydreaming and mind-wandering. Many scientists
believe the default network has two major hubs, one in the posterior
cingulate cortex with the precuneus and one in the medial prefrontal
cortex (shown above). Network overlay: Olaf Sporns/Indiana Univ.
(modified by J. Korenblat); Brain photo: Omikron/photoresearchersYou
may not be riding the latest social wave on Facebook or MySpace, or
tweeting your every impulse to fans on Twitter. But your brain is
hooked on networking.

Vision works because different brain regions link up to connect the
dots of light and color into a meaningful picture of the world.
Language depends on networks of neural circuitry that make sense of
the words you hear or see and that help you generate your side of the
conversation. Networks of nerves control the motion of your muscles,
allowing you to move smoothly and, when necessary, swiftly.

Networks are the “in” thing for brain scientists, as surely as they
have been for online social butterflies.

Scientists learn about the brain’s networks by asking people to
perform all sorts of mental acrobatics — interpreting optical
illusions, solving riddles, taking tests of mental or muscular skills.
But some neuroscientists think they can learn even more about the
brain by asking volunteers to just lie back, close their eyes and let
their minds wander.

Such unstructured journeys of the mind — be they planning tonight’s
dinner, thinking about that meeting at work and what your boss said
afterward, debating whether to drive or fly for your next vacation, or
recalling that day in your childhood when you first sat in your new
tree house listening to birds chirp —turn out to offer clues about one
of the most important, mysterious and well-connected networks of all.
It’s called the default mode network, and it’s responsible for what
the brain does when it is doing nothing in particular. It’s the
brain’s core, both physically and mentally, and it’s better connected
to the brain’s system of circuits than Kevin Bacon is to movie stars.

“I think the default mode network is the most exciting thing that has
happened in cognitive neuroscience in quite some time,” says Peter
Fransson, a neuroscientist at the Karolinska Institute in Stockholm.

Default brain settings may lead to daydreaming and mind-wandering, but
the network also conducts serious business. Neuroscientists still
hotly debate the network’s exact functions, however. Among its jobs
may be running life simulations, providing a sense of self and
maintaining crucial connections between brain cells. A few researchers
doubt the network is anything special at all.

But evidence suggests that a malfunctioning default network is
involved in diseases and disorders as diverse as Alzheimer’s disease,
autism, depression, post-traumatic stress disorder, Tourette syndrome,
amyotrophic lateral sclerosis, schizophrenia and attention-deficit/
hyperactivity disorder.

Busy behind the scenes


Despite its laid-back name, which neuroscientist Marcus Raichle coined
in a 2001 paper, the default mode network is one of the hardest-
working systems in the brain. It was discovered accidentally by
researchers watching the activity of brains at work on various tasks.

Neuroscientists use PET (short for positron emission tomography) and
functional MRI scanners to image and gauge brain activity. To tell
which areas of the brain become more active during a mental task,
scientists compare brain activity during the task with activity when
the person is at rest, either with eyes closed or while staring at a
dot or cross. Raichle, of Washington University in St. Louis, and
others saw that every time a person engaged in a mental activity such
as memorizing a list of words, a collection of brain regions
consistently decreased activity compared with their resting levels.
Only when people recall autobiographical memories or imagine
alternative situations is the network more active than it is at rest,
scientists have since found. (In this context, “rest” refers to a
state in which the brain is not engaged in a mental task but is still
monitoring the body and the world around it.) Raichle hypothesized
that the network is more active when the brain is at rest and has to
dial back its activity to let people concentrate on specific tasks.

Michael Greicius, a neurologist and neuroscientist at the Stanford
School of Medicine, put the resting part of Raichle’s 
theory to the
test. Greicius and his colleagues measured brain activity while
volunteers had their eyes closed and thought of nothing in particular.
The team used a technique called functional connectivity MRI to reveal
correlations in activity in different brain areas. The group reported
in 2003 that blood flow in parts of the brain implicated in the
default network rises and falls like the tides — in slow but
synchronized waves.

Enlarge
Network in syncAt top, fMRI images show areas of the brain that
coordinate activity when people are thinking about nothing in
particular. The left image shows the outside of a left-facing brain.
The right image shows the inner surface of one hemisphere of a right-
facing brain. Activity in the two major hubs of the default network
(indicated by yellow and orange arrows) rises and falls in time with
each other, as seen on a plot of those fluctuations below. Courtesy of
M. Raichle Those coordinated parts of the brain — with cumbersome
names such as the medial prefrontal cortex, posterior cingulate
cortex, retrosplenial cortex, precuneus, inferior parietal lobe and
hippocampus — are located mostly along the crevice separating the
brain’s hemispheres, and on each lobe behind and above the ears.
Researchers don’t agree on all the components of the default network,
but consensus is growing that it has two major hubs: the posterior
cingulate cortex, or PCC, with the precuneus, and the medial
prefrontal cortex.

Functions ascribed to those two areas may give clues to what the
default network is good for. The medial prefrontal cortex is involved
in imagining, thinking about yourself and “theory of mind,” which
encompasses the ability to figure out what others think, feel or
believe and to recognize that other people have different thoughts,
feelings and beliefs from you. The precuneus and PCC are involved in
pulling personal memories from the brain’s archives, visualizing
yourself doing various activities and describing yourself.

Together, these hubs give you a sense of who you are. Their prominence
in the network has led some researchers to propose that the function
of the default mode is to allow you to internally explore the world
and your place in it, so you can plot future actions, including
contingency plans for various scenarios you might encounter.

The network that never sleeps


Some scientists quibble with the name, but Fransson says the network
really is the brain’s default. Peter Williamson, a psychiatrist at the
University of Western Ontario in London, Canada, agrees.

“You don’t even have to be conscious for it to be apparent,” he says.

Slow yet coordinated fluctuations in activity bind the network
together. The syncopations continue even while people are asleep,
under anesthesia or in comas. But it is unlikely that such activity
reflects ongoing conscious processing, Greicius contends. The
fluctuations that move through the network are incredibly slow, he
says, with one cycle every 15 to 20 seconds. Most conscious thought
happens in split seconds, so it is more likely that the plodding
pulses are for “subconscious synapse maintenance,” he says.

Synapses are the connections between neurons where cell-to-cell
communication takes place. When two neurons stop “talking” to each
other, connections between them can be severed. Greicius thinks the
low-level fluctuations in the network help keep the neurons in
contact, sort of the brain-cell equivalent of Facebook status updates.

While it is good to stay connected, reverting to default isn’t always
helpful. The default mode network sometimes stirs during monotonous
tasks, drawing away a person’s attention. Such reactivation of the
network predicted errors up to 30 seconds before a person made a
mistake, Vince Calhoun of the MIND Research Network in Albuquerque and
colleagues reported in 2008 in the Proceedings of the National Academy
of Sciences. And a study published May 26 in that journal, by Kalina
Christoff of the University of British Columbia in Vancouver, Canada,
and colleagues, shows that not only is the default network involved in
mind-wandering, it also distracts executive areas of the brain, so
that people aren’t even aware that their minds have wandered off task.

Psychiatric connections


Researchers are also studying how defects in the coordination between
different parts of the default network may contribute to psychiatric
disorders. Calhoun, an electrical engineer at the University of New
Mexico, and colleagues at other institutions studied network activity
during a memory task in 115 people with schizophrenia and 130 healthy
people. Some subnetworks within the default mode network had trouble
disengaging in people with schizophrenia, impairing their ability to
focus on the task, the team reported online May 11 in Human Brain
Mapping.

Enlarge
Alzheimer's overlaps default networkAlzheimer's disease appears to
particularly affect brain areas involved in the default network. At
left, two views of the brain show areas activated (orange) when a
healthy person recalls personal memories. Amyloid plaques
characteristic of Alzheimer's accumulate (red) and the brain atrophies
(blue) in some of the same areas, as shown in scans of people with
dementia (middle and right). R.L. Buckner et al./Annals NY Academy of
Science 2008, reprinted with permission of Blackwell Publishing LTD;
M. Raichle (both)People with schizophrenia also have faster cycles of
activity in their default networks during a resting state than healthy
people do, Calhoun and another group of colleagues reported in the
March 2007 American Journal of Psychiatry.

Williamson and colleagues, meanwhile, have shown that the default
network’s connections with other parts of the brain may be important
in determining who develops PTSD after a traumatic event. People who
have been traumatized can become numb and lose their sense of self,
Williamson says. The researchers examined default networks in women
who developed PTSD after trauma in childhood. The study found altered
levels of connectivity among parts of the default network as well as
between the network and other parts of the brain. The findings,
published in May in the Journal of Psychiatry & Neuroscience, could
indicate that trauma creates disturbances in the network’s ability to
create a sense of self.

The default network may also be the launching point for Alzheimer’s
disease’s assault on the brain. The characteristic plaques of the
disease deposit preferentially in the brain regions most associated
with the network, studies have shown. And Greicius and his colleagues
reported online last year in the June 2008 PLoS Computational Biology
that activity in the default network is affected by the disease.

At least one study suggests that the default network may be vulnerable
to Alzheimer’s disease decades before symptoms or plaques show up.
Young people who carry a genetic risk factor for the disease have more
activity in the default network, particularly in the hippocampus, than
young people who don’t have the genetic risk, researchers from Oxford
University and Imperial College in England reported in the April 28
Proceedings of the National Academy of Sciences. The authors say the
study provides evidence for the theory that the default network’s
constantly high activity eventually burns it out, leaving it
vulnerable to Alzheimer’s disease.

Greicius says he isn’t a fan of this “use it and lose it” theory.
Other networks in the brain also burn a lot of energy, even at rest,
but they don’t fall prey to Alzheimer’s disease. Instead, Alzheimer’s
and four other neurodegenerative diseases each target a different
brain network, Greicius and colleagues including William Seeley of the
University of California, San Francisco discovered. The results,
published April 16 in Neuron, could mean that neurons that fire
together die together. The researchers don’t yet understand why. It
could be that when a neuron dies, its silence triggers death in
neighboring neurons, or neuron-killing substances might pass from one
cell to another through 
synapses, Greicius says.

Blueprint for the brain


To understand what goes wrong with the default network to lead to
psychiatric disorders, scientists need to understand how the network
is built. Assembling disparate regions of the brain into a
coordinated, coherent system surely is no simple task for the
developing brain.

“One might imagine that the development of self might take a bit of
time to sculpt,” Raichle says.

Only a few studies have been done with children, so the picture of the
nascent default network is about as clear as an ultrasound image is to
someone other than an expectant parent. But new ways of analyzing
neural connections are bringing the picture into better focus.

Fransson and his colleagues used fMRI to scan the brains of sleeping
premature infants who had reached the equivalent of 40 weeks of
gestation to see whether the default network is already in place when
babies are first born. The researchers could not find evidence that
the default mode is operational in newborns, although five other brain
networks are already online, the team reported in 2007. Fransson says
he is not surprised that newborns lack a network that draws on
personal experience and dreams of what is to come.

“Infants cannot plan for their futures,” he says. “They don’t think
about their pasts.”

But a recent study by Weili Lin, a 
neuroscientist at the University
of North Carolina at Chapel Hill, and colleagues shows that infants as
young as 2 weeks have rudimentary, incomplete default mode networks.
The study, published in the April 21 Proceedings of the National
Academy of Sciences, tracks development of the network from shortly
after birth into toddlerhood. Newborns’ default networks connect six
brain regions, Lin’s group found.

It doesn’t take long for the brain to develop a default mode, Lin
showed. By age 1, babies link 13 brain regions in their default
network, including 10 parts found in the adult network. In 2-year-
olds, the default network is even bigger, comprising 19 regions, 13
consistent with the network in adults. But bigger networks can also be
inefficient, Lin says, noting that adult default networks have been
pruned of extraneous connections.

Lin and his colleagues are continuing to scan many of the children in
the study as they age to track how the normal brain develops.
Preliminary data from 4-year-olds indicate that extra connections are
severed as the brain ages, he says.

A group of researchers at Washington University including Raichle,
Steven Petersen, Bradley Schlaggar and Damien Fair (now at the Oregon
Health & Science University) are piecing together the network’s
development from age 7 into early adulthood.

Brain connections in 7-year-olds are organized differently than in
adults. Children have more short-range connections among neighboring
brain regions and fewer long-range connections, particularly among the
parts of the default network in the back and front, the team reported
last year in the Proceedings of the National Academy of Sciences. As
children age, the connections are rewired. Adolescents have a network
structure somewhere between that of elementary-age children and
adults.

“It’s like different cliques of friends in childhood break up and
create different cliques in adulthood,” Petersen says.

Given the lack of long-range connections in children’s brains,
researchers were surprised to discover that kids’ default networks
aren’t clunky. The team mapped how the brain makes connections in the
network, a neuroscience version of the game to link actor Kevin Bacon
to other actors in Hollywood through people with film appearances in
common. Fewer steps means more efficient connections. While children’s
connections are structured differently, they have enough shortcuts to
make information transfer in the network just as efficient as in
adults, the 
scientists reported online May 1 in PLoS Computational
Biology.

Once people reach adulthood, activity in the network is fairly
consistent from person to person, with some slight differences between
the sexes and in older versus younger people, Williamson and his
colleagues wrote in a 2008 paper in NeuroReport.

This consistency in the network from person to person is remarkable,
especially considering what its function is supposed to be. Everyone’s
brain is thinking different thoughts while in the default mode, Fair
says, and yet all healthy brains in default mode look essentially
alike.

Such fundamental issues are among the puzzles of the default network
remaining to be solved.

“Nobody has really figured out what it is and what it does,”
Williamson says. “But somebody will.”



--------------------------------------------------------------------------------


Wandering and wondering


The brain’s default mode network — a series of connected areas that
work hardest when most of the brain is at rest —is active during
daydreaming and mind-wandering. The network may also be involved in
imagining how certain situations play out and in giving people their
sense of self and where they fit in the world. Many scientists believe
the default network has two major hubs, one in the posterior cingulate
cortex with the precuneus and one in the medial prefrontal cortex. The
two hubs are highlighted in the image above, which shows a human brain
viewed from the top and overlaid with a computer-generated map showing
the most robust of the network’s structural connections. Also visible
in the map are the left and right inferior parietal lobes, which are
among a number of other brain regions involved in the default network.



--------------------------------------------------------------------------------


Network in sync


At top, fMRI images show areas of the brain that coordinate activity
when people are thinking about nothing in particular. The left image
shows the outside of a left-facing brain. The right image shows the
inner surface of one hemisphere of a right-facing brain. Activity in
the two major hubs of the default network (indicated by yellow and
orange arrows) rises and falls in time with each other, as seen on a
plot of those fluctuations below.



--------------------------------------------------------------------------------


Alzheimer’s overlaps default network


Alzheimer’s disease appears to particularly affect brain areas
involved in the default network. At left, two views of the brain show
areas activated (orange) when a healthy person recalls personal
memories. Amyloid plaques characteristic of Alzheimer’s accumulate
(red) and the brain atrophies (blue) in some of the same areas, as
shown in scans of people with dementia (middle and right).

.