That’s what Malia Mason of Dartmouth College in Hanover, New Hampshire, wanted to know. Working with colleagues at Harvard and the University of Aberdeen in Scotland, Mason (now at Massachusetts General Hospital) used fMRI to find the neural correlates of daydreaming. The researchers found that when people are mentally adrift, brain activity resides mainly in the default network, a tightly connected but distinct set of cortical regions that are active when the brain is not engaged in any specific task. The results appeared in the January 18 Science.

The default network is familiar to AD researchers from Randy Buckner and colleagues’ recent work showing that the distribution of both amyloid plaque and brain atrophy in Alzheimer disease overlaps with this network (see ARF related news story; see also Greicius et al., 2004). The default network calls heavily on recent memories, which could help explain the selective effects of AD on memory functions.

Mason and colleagues used a tried-and-true method to set the mind of their subjects adrift, that is, boredom. They trained volunteers on monotonous recognition tasks, and that led to a high rate of mind wandering as reported by the volunteers. In MRI studies, these dull tasks elicited activity in their default networks, which included parts of the posterior cingulate and precuneus regions, the posterior lateral cortices, the insular cortices, the cingulate, and parts of the medial prefrontal cortex.

The people who reported the most mental meandering during these tasks had the most active default networks as measured by fMRI. When they faced a more challenging task that required them to focus, activity in their default network subsided. The results suggest that default network activity produces a kind of basal mental state that kicks in when the mind is not actively engaged.

Not everyone spends the same amount of time in this basal state; people show stable differences in their tendency to muse. The researchers correlated the intensity of fMRI activation signals of the default network in different volunteers with their scores on a standardized test of daydreaming frequency. Given the association reported by Buckner and colleagues among the default network, amyloid plaque deposition, and brain atrophy, could it be that differences in this network activity might somehow lead to differences in the risk for AD? Now there’s something on which to concentrate the mind.—Pat McCaffrey.

Reference:
Mason MF, Norton MI, Van Horn JD, Wegner D, Grafton ST, Macrae CN. Wandering minds: The default network and stimulus-independent thought. Science. 19 Jan 2007; 315:393-395. Abstract