The Brain’s Dark Energy


Brain regions active when our minds wander may hold a key to  understanding neurological disorders and even consciousness itself

KEY CONCEPTS
• Neuroscientists have long  thought that the brain’s circuits are turned off
when a person is at rest.
• Imaging experiments,however, have shown that  there is a persistent level
of background activity.
• This default mode, as it is  called, may be critical in
planning future actions.
• Miswiring of brain regions  involved in the default  mode may lead to disorders
ranging from Alzheimer’s  to schizophrenia.

image

Imagine  you are almost dozing in a lounge chair  outside, with a magazine on your lap. Suddenly,a fly lands on your arm. You grab the magazine  and swat at the insect. What was going on in your brain after the fly landed? And what was
going on just before?

Many neuroscientists have long assumed that much of the neural activity inside
your head when at rest matches your subdued,somnolent mood. In this view, the activity
in the resting brain represents nothing more than random noise, akin to the snowy pattern on the
television screen when a station is not broadcasting.
Then, when the fly alights on your forearm,he brain focuses on the conscious task of squash.
ing the bug. But recent analysis produced by neuro imaging  technologies has revealed something
quite remarkable: a great deal of meaningful activity  is occurring in the brain when a person is
sitting back and doing nothing at all.
It turns out that when your mind is at rest when you are daydreaming quietly in a chair,
say, asleep in a bed or anesthetized for surgery dispersed brain areas are chattering away to one
another. And the energy consumed by this ever  active messaging, known as the brain’s default
mode, is about 20 times that used by the brain  when it responds consciously to a pesky fly or
another outside stimulus. Indeed, most things we do consciously, be it sitting down to eat dinner
or making a speech, mark a departure from  the baseline activity of the brain default mode.
Key to an understanding of the brain’s default  mode has been the discovery of a heretofore unrecognized
brain system that has been dubbed  the brain’s default mode network (DMN). The
exact role of the DMN in organizing neural activity  is still under study, but it may orchestrate
the way the brain organizes memories and various  systems that need preparation for future
events: the brain’s motor system has to be revved  and ready when you feel the tickle of a fly on
your arm.

The DMN may playa critical role in  synchronizing all parts of the brain so that, like
racers in a track competition, they are all in the  proper “set” mode when the starting gun goes
off. If the DMN does prepare the brain for conscious  activity, investigations of its behavior may
provide clues to the nature of conscious experience.
Neuroscientists have reason to suspect,moreover, that disruptions to the DMN may underlie
simple mental errors as well as a range of  complex brain disorders, from Alzheimer’s disease
to depression.

Probing Dark Energy
The idea that the brain could be constantly busy  is not new. An early proponent of that notion
was Hans Berger, inventor of the familiar electroencephalogram,which records electrical
activity in the brain with a set of wavy lines on a  graph. In seminal papers on his findings, pub-
lished in 1929, Berger deduced from the ceaseless  electrical oscillations detected by the device
that “we have to assume that the central nervous  system is always, and not only during wakefulness,
in a state of considerable activity.”
But his ideas about how the brain functions  were largely ignored, even after noninvasive imaging
methods became a fixture in neuroscience  laboratories. First, in the late 1970s, came positron-
emission tomography (PET), which measures  glucose metabolism, blood flow and oxygen
uptake as a proxy for the extent of neuronal  activity, followed in 1992 by functional magnetic
resonance imaging (fMRI), which gauges brain  oxygenation for the same purpose. These technologies
are more than capable of assaying brain  activity, whether focused or not, but the design
of most studies inadvertently led to the impression  that most brain areas stay pretty quiet until
called on to carry out some specific task.
Typically neuroscientists who run imaging  experiments are trying to pinpoint the brain regions
that give rise to a given perception or behavior.
The best study designs for defining such  regions simply compare brain activity during
two related conditions. If researchers wanted to  see which brain areas are important during reading
words aloud (the “test” condition) as opposed  to viewing the same words silently (the
“control” condition), for instance, they would  look for differences in images of those two conditions.
And to see those differences clearly, they  would essentially subtract the pixels in the passive-
reading images from those in the vocal image;
activity of neurons in the areas that remain  “lit up” would be assumed to be the ones necessary
for reading aloud. Any of what is called intrinsic  activity, the constant background activity,
would be left on the cutting-room floor. Representing  data in this way makes it easy to
envision areas of the brain being “turned on,”during a given behavior, as if they were inactive
until needed by a particular task.
Over the years, however, our group, and others,became curious about what was happening
when someone was simply resting and just letting  the mind wander. This interest arose from
a set of hints from various studies that suggested  the extent of this behind-the-scenes activity.
One clue came from mere visual inspections  of the images. The pictures showed that areas in
many regions of the brain were quite busy in  both the test and the control conditions. In part
because of this shared background “noise,” differentiating  a task from the control state by looking at the separate raw images is difficult if not  impossible and can be achieved only by applying  sophisticated computerized image analysis.
Further analyses indicated that performing a  particular task increases the brain’s energy consumption
by less than 5 percent of the underlying  baseline activity. A large fraction of the overall
activity-from 60 to 80 percent of all energy  used by the brain-occurs in circuits unrelated
to any external event. With a nod to our astronomer  colleagues, our group came to call this intrinsic
activity the brain’s dark energy, a reference  to the unseen energy that also represents
the mass of most of the universe.
The question of the existence of neural dark  energy also arose when observing just how little
information from the senses actually reaches  the brain’s internal processing areas. Visual information,
for instance, degrades significantly  as it passes from the eye to the visual cortex.
Of the virtually unlimited information available  in the world around us, the equivalent of 10
billion bits per second arrives on the retina at  the back of the eye. Because the optic nerve attached
to the retina has only a million output  connections, just six million bits per second can
leave the retina, and only 10,000 bits per second  make it to the visual cortex.
After further processing, visual information  feeds into the brain regions responsible for forming
our conscious perception. Surprisingly, the  amount of information constituting that conscious
perception is less than 100 bits per second.
Such a thin stream of data probably could  not produce a perception if that were all the
brain took into account; the intrinsic activity  must playa role.
Yet another indication of the brain’s intrinsic  processing power comes from counting the number
of synapses, the contact points between neurons.
In the visual cortex, the number of synapses  devoted to incoming visual information is less
than 10 percent of those present. Thus, the vast  majority must represent internal connections
among neurons in that brain region.

Discovering the Default Mode
These hints of the brain’s inner life were well  established. But some understanding was needed
of the physiology of the brain’s intrinsic activity  and how it might influence perception and
behavior. Happily, a chance and puzzling observation  made during PET studies, later corroborated
with fMRI, set us on a path to discovering  the DMN.

In the mid-1990s we noticed quite by accident  that, surprisingly, certain brain regions experienced
a decreased level of activity from the baseline   resting state when subjects carried out some
task. These areas-in particular, a section of the  medial parietal cortex (a region near the middle
of the brain involved with remembering personal  events in one’s life, among other things)-registered
this drop when other areas were engaged  in carrying out a defined task such as reading
aloud. Befuddled, we labeled the area showing  the most depression MMPA, for “medial mystery
parietal area.”
A series of PET experiments then confirmed  that the brain is far from idling when not engaged
in a conscious activity. In fact, the MMP A  as well as most other areas remains constantly
active until the brain focuses on some novel task,at which time some areas of intrinsic activity decrease.
At first, our studies met with some skepticism.
In 1998 we even had a paper on such findings  rejected because one referee suggested that
the reported decrease in activity was an error in  our data. The circuits, the reviewer asserted,
were actually being switched on at rest and  switched off during the task. Other researchers,
however, reproduced our results for both the  medial parietal cortex-and the medial prefrontal
cortex (involved with imagining what other  people are thinking as well as aspects of our
emotional state). Both areas are now considered  major hubs of the DMN.
The discovery of the DMN provided us with  a new way of considering the brain’s intrinsic activity.
Until these publications, neurophysiologists  had never thought of these regions as a system
in the way we think of the visual or motor  system-as a set of discrete areas that communicate
with one another to get a job done. The idea  that the brain might exhibit such internal activity
across multiple regions while at rest had escaped  the neuroimaging establishment. Did the
DMN alone exhibit this property, or did it exist  more generally throughout the brain? A surprising
finding in the way we understand and analyze  fMRI provided the opening we needed to
answer such questions.
The fMRI signal is usually referred to as the  blood oxygen level-dependent, or BOLD, signal
because the imaging method relies on changes in  the level of oxygen in the human brain induced
by alterations in blood flow. The BOLD signal  from any area of the brain, when observed in a
state of quiet repose, fluctuates slowly with cycles  occurring roughly every 10 seconds. Fluctuations this slow were considered to be mere noise,and so the data detected by the scanner were simply  eliminated to better resolve the brain activity
for the particular task being imaged.
The wisdom of discarding the low-frequency  signals came into question in 1995, when Bharat
Biswal and his colleagues at the Medical College  of Wisconsin observed that even while a subject
remained motionless, the “noise” in the area of  the brain that controls right-hand movement fluctuated
in unison with similar activity in the area  on the opposite side of the brain associated with
left-hand movement. In the early 2000s Michael  Greicius and his co-workers at Stanford University
found the same synchronized fluctuations in  the DMN in a resting subject.
Because of the rapidly accelerating interest in  the DMN’s role in brain function, the finding by
the Greicius group stimulated a flurry of activity  in laboratories worldwide, including ours, in
which all of the noise, the intrinsic activity of the  major brain systems, was mapped. These remarkable
patterns of activity appeared even under  general anesthesia and during light sleep, a
suggestion that they were a fundamental facet of  brain functioning and not merely noise.
It became clear from this work that the DMN  is responsible for only a part, albeit a critical
part, of the overall intrinsic activity-and the  notion of a default mode of brain function extends
to all brain systems. In our lab, discovery  of a generalized default mode came from first examining
research on brain electrical activity  known as slow cortical potentials (SCPs), in  which groups of neurons fire every 10 seconds or
so. Our research determined that the spontaneous fluctuations observed in the BOLD images
were identical to SCPs:the same activity detected  with different sensing methods.
We then went on to examine the purpose of  SCPs as they relate to other neural electrical signals.
As Berger first showed and countless others  have since confirmed, brain signaling consists
of a broad spectrum of frequencies, ranging  from the low-frequency SCPs through activity in
excess of 100 cycles per second. One of the great  challenges in neuroscience is to understand how
the different frequency signals interact.

It turns out that SCPs have an influential role.
Both our own work and that of others demonstrate  that electrical activity at frequencies above
that of the SCPs synchronizes with the oscillations,or phases, of the SCPs. As observed recently
by Matias Paiva and his colleagues at the  University of Helsinki, the rising phase of an
SCP produces an increase in the activity of signals  at other frequencies.
The symphony orchestra provides an apt metaphor,with its integrated tapestry of sound arising
from multiple instruments playing to the  same rhythm. The SCPs are the equivalent of the
conductor’s baton. Instead of keeping time for a  collection of musical instruments, these signals
coordinate access that each brain system requires  to the vast storehouse of memories and
other information needed for survival in a complex,ever changing world. The SCPs ensure that
the right computations occur in a coordinated fashion at exactly the correct moment.
But the brain is more complex than a symphony  orchestra. Each specialized brain system one
that controls visual activity, another that actuates  muscles-exhibits its own pattern of
SCPs. Chaos is averted because all systems are   not created equal. Electrical signaling from some
brain areas takes precedence over others. At the  top of this hierarchy resides the DMN, which
acts as an tiber-conductor to ensure that the cacophony  of competing signals from one system
do not interfere with those from another. This  organizational structure is not surprising, because
the brain is not a free-for-all among independent  systems but a federation of interdependent
components.
At the same time, this intricate internal activity  must sometimes give way to the demands of
the outside world. To make this accommodation,SCPs in the DMN diminish when vigilance
is required because of novel or unexpected sensory  inputs: you suddenly realize that you promised
to pick up a carton of milk on the drive  home from work. The internal SCP messaging
revives once the need for focused attention dwindles.
The brain continuously wrestles with the  need to balance planned responses and the immediate
needs of the moment.

Consciousness and Disease
The ups and downs of the DMN may provide  insight into some of the brain’s deepest mysteries.
It has already furnished scientists with fascinating  insights into the nature of attention, a
fundamental component of conscious activity.
In 2008 a multinational team of researchers  reported that by watching the DMN, they could
tell up to 30 seconds before a subject in a scanner  was about to commit an error in a computer
test. A mistake would occur if, at that time, the  default network took over and activity in areas
involved with focused concentration abated.
And in years to come, the brain’s dark energy  may provide clues to the nature of consciousness.
As most neuroscientists acknowledge, our  conscious interactions with the world are just a
small part of the brain’s activity. What goes on  below the level of awareness-the brain’s dark
energy, for one-is critical in providing the context  for what we experience in the small window
of conscious awareness.
Beyond offering a glimpse of the behind-the scenes  events that underlie everyday experience,
study of the brain’s dark energy may provide  new leads for understanding major neurological
maladies. Mental gymnastics or intricate movements  will not be required to complete the exercise.
A subject need only remain still within the  scanner while the DMN and other hubs of dark
energy whir silently through their paces.
Already this type of research has shed new   light on disease. Brain-imaging studies have
found altered connections among brains cells in  the DMN regions of patients with Alzheimer’s,
depression, autism and even schizophrenia. Alzheimer’s,in fact, may one day be characterized
as a disease of the_DMN. A projection of the  brain regions affected by Alzheimer’s fits neatly
over a map of the areas that make up the DMN.
Such patterns may not only serve as biological  markers for diagnosis but may also provide deeper
insights into causes of the disease and treatment  strategies.
Looking ahead, investigators must now try to  glean how coordinated activity among and within
brain systems operates at the level of the individual  cells and how the DMN causes chemical
and electrical signals to be transmitted through  brain circuits. New theories will then be needed
to integrate data on cells, circuits and entire neural  systems to produce a broader picture of how
the brain’s default mode of function serves as a  master organizer of its dark energy. Over time
neural dark energy may ultimately be revealed  as the very essence of what makes us tick .•

by Marcus E. Raichle

is professor  of radiology and neurology at the Washington University School of
Medicine in St. louis. For many  years Raichle has led a team that  investigates human brain function  using positron-emission tomography  and functional magnetic  resonance imaging. He was elected  to the Institute of Medicine in 1992
and to the National Academy of  Sciences in 1996,

image

MORE TO  EXPLORE
Spontaneous Fluctuations in Brain  Activity Observed with Functional
Magnetic Resonance Imaging.
Michael D. Fox and Marcus E. Raichle in Nature Reviews Neuroscience, Vol.
8, pages 700-711; September 2007.
Disease and the Brain’s Dark Energy.
Dongyang Zhang and Marcus E.  Raichle in Nature Reviews Neurology,
Vol. 6, pages 15-18; January 2010.
Two Views of Brain Function.
Marcus E. Raichle in Trends in Cognitive Science (in press).

SOURCE  SCIENTFIC AMERICAN 2010

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About sooteris kyritsis

Job title: (f)PHELLOW OF SOPHIA Profession: RESEARCHER Company: ANTHROOPISMOS Favorite quote: "ITS TIME FOR KOSMOPOLITANS(=HELLINES) TO FLY IN SPACE." Interested in: Activity Partners, Friends Fashion: Classic Humor: Friendly Places lived: EN THE HIGHLANDS OF KOSMOS THROUGH THE DARKNESS OF AMENTHE
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One Response to The Brain’s Dark Energy

  1. Reblogged this on controlledfreak and commented:
    So cool.

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