Session 8: Neuroimaging of the default mode

Fox et al. (2005)* described a pair of anticorrelated networks of brain structures in connection with attention-demanding cognitive tasks: a task-positive network that increases activity during attention to the task and a task-negative network including the default network that decreases activity during attention to the task. The other articles for this session attempt to establish what the default network does when it becomes active. Mason et al. (2007)** found a correlation between increased activity in the default network and both more practiced tasks that allow for more mind wandering and individuals’ self-reported tendency to mind wander. Andrews-Hanna et al. (2010)*** used fMRI imaging during tasks requiring different levels of visual attention and self-reports of mind wandering on the task to determine whether default network activity is connected with spontaneous (particularly internal) cognition (i.e., mind wandering) or a broadening of external attention. They concluded that default network activity is associated with spontaneous cognition, not broadening of external attention (Andrews-Hanna et al., 2010). Stawarczyk et al. (2011)**** categorized thoughts during a task by both task-relatedness and stimulus-dependency to identify whether and how default network activity is associated with task-unrelated and stimulus-independent thoughts. They found evidence for default network involvement in both categories of thought, and they found that the activation associated with task-unrelated and stimulus-independent thoughts is additive, such that mind-wandering (task-unrelated and stimulus-independent thought) is associated with greater activity in some areas of the default network than either task-unrelated but stimulus-dependent (i.e., outside distractions) or stimulus-independent but task-related (i.e., evaluating task difficulty or performance) thoughts are (Stawarczyk et al., 2011).

The explanations for default network activity provided in Mason et al. (2007), Andrews-Hanna et al. (2010), and Stawarczyk et al. (2011) build on Fox et al.’s (2005) findings and are consistent with them: Fox et al. (2005) described what the default network does during attention to an external task (i.e., it deactivates). The other studies described what the default network does when attention is not focused on performing the external task.

Andrews-Hanna et al. (2010) and Stawarczyk et al. (2011) gave what initially seem like contradictory explanations for default network activity: Andrews-Hanna et al. (2010) suggested that default network activity is not associated with external focus characteristics, while Stawarczyk et al. (2011) linked some default network activity to attention to external task-unrelated stimuli. But Andrews-Hanna et al. (2010) specifically observed that their study was limited to external stimuli within the same visual display and that default network activity could actually be associated with broadening of attention to a much larger scale, such as visual attention to other parts of the room or attention in some other modality such as sound or smell. And Stawarczyk et al. (2011) did not have their participants identify the specific external stimuli they were distracted by during task-unrelated stimulus-dependent thoughts. So the two sets of findings could be consistent in that both internal and stimulus-dependent thoughts can increase default network activity, provided that the stimulus triggering the stimulus-dependent thoughts comes from a focus broader than the screen the primary task is on or the headphones or speakers some task-related auditory stimulus is coming from.

Because the Stawarczyk et al. (2011) study monitored participants’ thoughts during the fMRI scans—and particularly because it used thought probes during the tasks/scans—it seems to provide a particularly valid correlation between actual thought content and default network activity. Andrews-Hanna et al. (2010) did not use real-time thought profiles, but they did create conditions that specifically differed in the scope of external attention required, so their results are convincing in the context of whether default network activity tends to be associated more with a broad external focus or mind-wandering. And they also had participants provide post-task self reports of their thoughts during the task (Andrews-Hanna et al., 2010), which might not result in the most accurate information but does at least provide some information specific to the participants and tasks represented in each brain scan.

By contrast, Mason et al.’s (2007) evidence for the connection between default network activity and mind-wandering seems awfully indirect: they did associate increased default network activity with practiced tasks, which are associated with more mind wandering, and also with their participants’ self-reported general mind wandering tendencies, but they did not actually monitor the participants’ thoughts while they were performing the tasks in the scanner either through thought probes during the tasks or through post-task self reports. This seems like a major weakness for correlating brain activity to thought content.

* Fox, M. D., Snyder, A. Z., Vincent, J.L., Corbetta, M., Van Essen, D. C., & Raichle, M. E. (2005). The human brain is intrinsically organized into dynamic, anticorrelated functional networks. PNAS, 102, 9673-9678.

** Mason, M. F., Norton, M. I., Van Horn, J. D., Wegner, D. M., Grafton, S. T., & Macrae, C. N. (2007). Wandering minds: the default network and stimulus-independent thought. Science, 315, 393-395.

*** Andrews-Hanna, J. R., Reidler, J. S., Huang, C., & Buckner, R. L. (2010). Evidence for the default network's role in spontaneous cognition. Journal of Neurophysiology, 104, 322-335.

**** Stawarczyk, D., Majerus, S., Maquet, P., & D'Argembeau, A. (2011). Neural correlates of ongoing conscious experience: Both task-unrelatedness and stimulus-independence are related to default network activity. PLoS ONE, 6(2): e16997.