Abstract
On a regional scale the brain is organized into dynamic functional networks. The activity within one of these, the default network, can be dissociated from that in other task-specific networks. All brain networks are connected structurally but evidently are only transiently connected functionally. One hypothesis as to how such transient functional coupling occurs is that network formation and dissolution is mediated by increases and decreases in oscillatory synchronization between constituent brain regions. If so, then we should be able to find transient differences in intra-network synchronization between the default network and a task-specific network. In order to investigate this hypothesis we conducted two experiments in which subjects engaged in a Sustained Attention to Response Task while having brain activity recorded via high-density electroencephalography (EEG). We found that during periods when attention was focused internally (mind wandering) there was significantly more neural phase synchronization between brain regions associated with the default network, whereas during periods when subjects were focused on performing the visual task there was significantly more neural phase synchrony within a task-specific brain network that shared some of the same brain regions. These differences in network synchrony occurred in each of theta, alpha, and gamma frequency bands. A similar pattern of differential oscillatory power changes, indicating modulation of local synchronization by attention state, was also found. These results provide further evidence that the human brain is intrinsically organized into default and task-specific brain networks, and confirm that oscillatory synchronization is a potential mechanism for functional coupling within these networks.
Highlights
At a regional scale the brain is organized into functionally specific networks (Passingham et al, 2002; Bullmore and Sporns, 2009)
If inter-regional phase synchronization is a plausible mechanism for the transient formation and dissolution of default and task-specific functional networks, phase synchronization within task-specific networks should be greater when attention is focused on a specific task, whereas that within the default network should be greater when attention is focused inward, away from the external task
We used a one-tailed test in this case because we only wished to confirm that our Sustained Attention to Response Task (SART) results were in the typical direction
Summary
At a regional scale the brain is organized into functionally specific networks (Passingham et al, 2002; Bullmore and Sporns, 2009). The dominant brain regions comprising the default network – the medial-frontal cortex, the temporal lobe, the hippocampal formation, and the parietal lobe (Buckner et al, 2007) have been shown to be functionally linked (Greicius et al, 2003, 2009; Honey et al, 2009). It is still unknown, how the brain switches functionally between default and task-specific networks, or whether default and task-specific networks are always anticorrelated or can be simultaneously active. To expect extreme anticorrelation of synchronization in these networks if a task is ongoing and performed at a reasonable level, as synchronization in the task-specific network would be required to maintain performance if the synchronization is functionally important
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