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 can't daydream.

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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