Some people seem to continually have their heads in the clouds. Perhaps they are pondering during their drive to work the next pickle 24 protagonist Jack Bauer will find himself in. Or maybe they are assessing while buttering toast the Indianapolis Colts' chances of finally making it to the Super Bowl. Or considering where they will dine that evening as they tap out an e-mail. The question is: What makes their minds veer from the task at hand?

Researchers at Dartmouth College may have the answer. They found that a default network of regions in the brain's cortex—a grouping known to be active when the mind is completely unoccupied—is firing away as a person is engaged in routine activities. Malia Mason, now a postdoctoral researcher of neurocognition at Harvard Medical School, trained subjects in verbal and spatial memory tasks that after four days of continual repetition became quite banal—perfect conditions for thinking about something unassociated with the work at hand. In fact, subjects reported more daydreaming when performing the rehearsed sequences rather then when the tasks were tweaked slightly to introduce a novel stimulus requiring a bit more focus.

On the fifth day, the subjects performed these activities while being surveyed by functional magnetic resonance imaging (fMRI). While the subjects were not performing any task, there was activation in several cortical regions, including parts of the medial prefrontal cortex (involved in executive functions), the premotor cortex (which coordinates body movements), and the cingulate (part of the limbic system that is implicated in memory and learning). When the subjects were asked to perform their well-rehearsed tasks, many of these areas were recruited once again, but when the job was slightly altered, the signals from these areas attenuated.

This finding "suggests that the default network appears to be associated with the production of these thoughts," Mason says.

The research team speculates that when engaged in a mundane task, mind wandering allows people to remain properly aroused. Alternatively, they say, daydreams could be a conduit for uniting experiences from a person's past or present to their future. Or, the brain may just have evolved the ability to handle more than one function at once.

"In a sense, these thoughts reflect an amazing capacity on our part to multitask," Mason explains, expanding on the last possibility. "It's like we have a sense of what we can and what we cannot get away with. In other words, it is as if we have a sense of how much attentional resources we have "left over" and [then we] allocate these resources to working out some problem or anticipating what we have to do in the near future."

Jonathan Schooler, a psychologist at the University of British Columbia, says that Mason's study illustrates that daydreaming is really the default state of the brain. He cautions, however, that while he finds the evidence—published in this week's Science—compelling, the measurements are indirect. "They didn't actually examine activation of the default network specifically during times when individuals were reporting mind wandering versus not," he says. "That's an important additional line of research that needs to be done."