delanceyplace.com 2/22/10 - the brain's dark matter

In today's excerpt - the mind 'at rest' is
often more
active, and at the least almost as active, as
the mind
when it is engaged in a task:

"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. But recent analysis produced by
neuroimaging 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, [or]
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
an 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. ...

"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. ...

"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 experiments then confirmed that
the brain
is far from idling when not engaged in a
conscious
activity. In fact, the MMPA 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
brain's default mode network."

Marcus E. Raichle, "The Brain's Dark Energy,"
Scientific American, March 2010, pp.
44-47.