Appears to Have
NEW YORK (Reuters Health) - Much as
a car's engine hums along even when it's parked in neutral, the
brain seems to contain a "default mode" in which certain regions
become more active at rest, US researchers report.
"During rest, these regions appear
to be interacting, because they change at similar rates," lead
author Dr. Michael D. Greicius of Stanford University in California
told Reuters Health.
Intriguingly, the behavior of these
brain regions bears a certain resemblance to what one would expect
from brain areas that make up human consciousness, Greicius added.
The default mode network supported
in the current study generally increases its activity when the
brain is at rest, then drops in activity once people are called
to a certain task. In a similar way, Greicius said, a person could
be daydreaming or following a stream of consciousness, but those
activities would be zapped away as soon as the person was called
to action, perhaps by a ringing telephone.
In addition, some of the brain
regions that may form parts of the so-called default mode network
have shown in previous studies to be involved in certain aspects
of consciousness, Greicius added. For instance, one of the brain
regions looked at in the current study, the posterior cingulate
cortex (PCC), has been shown to play a role in the brain processes
by which people recall memories. In addition, PCC activity tends
to peter out as people lose consciousness when sedated.
Given that PCC forms a part of
the theoretical default mode network, Greicius and his colleagues
suggest that this network may serve similar functions, such as
involving itself in remembering past events, mulling over information,
and thinking about the future.
Greicius cautioned in an interview,
however, that the results of the current study do not demonstrate
that the default mode does, in fact, represent the brain network
involved in consciousness, nor is it involved in any specific
activity related to consciousness.
"All of this stuff is supportive,
but none of it is definitive," he said.
During the study, Greicius and
his colleagues measured brain activity in a handful of people
during different activities: performing a mental task, passively
watching a pattern on a screen, or resting with their eyes closed.
They report their findings in the online Early Edition of Proceedings
of the National Academy of Sciences.
The authors found that PCC and
at least one other brain region known as the ventral anterior
cingulate cortex seem to show similar rates of increase and decrease
in activity in response to what people are doing.
Greicius noted that the default
mode regions continued to show high activity while people passively
watched the screen pattern--a finding that makes sense, he said,
for just because you are looking at something, doesn't mean you
The current findings also demonstrate
that the brain may need to be active even when the mind is at
rest, he added. "When the car is running, the engine is still
purring," Greicius said.
SOURCE: Proceedings of the National
Academy of Sciences 2002;10.1073/pnas.0135058100.
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