Default Mode Network Activation

These readings examine the role of the default mode network in attention-demanding tasks and associated mind-wandering.

Fox et al. (2005) analyze brain activity in the presence of attention-demanding cognitive tasks. They found that different regions increase in activity (task-positive network) while others decrease (task-negative network) and that this activation/deactivation dichotomy is observed when the brain is at rest, i.e. in the absence of an overt task. Their work therefore supports the model of both correlated and anticorrelated networks associated with the default network. 

Mason et al. (2007), Andrews-Hanna et al. (2010), and Stawarczyk et al. (2011) build on the findings of Fox et al. about the activation of the default mode network during mind-wandering experiences.

In an attempt to understand why and when mind-wandering affects default network activity, Mason et al. (2007) used a thought-sampling method and fMRI imaging to examine which regions of the default network were activated during mind-wandering. Instead of solely comparing SIT production and default network activation between a state of rest versus during a demanding task, they examine such effects during tasks varying in degree of mind-wandering likelihood, depending on whether a task is novel or practiced.  Extending on previous findings, they observe additional cortical areas implicated in the default network, specifically those of the premotor and supplementary motor cortex, which had not been included by Fox et al. (2005). If Mason et al.’s results are indeed correct, in that these regions are active at a resting state, one would expect that Fox et al. would have found them active only in the task-negative network. However, instead Fox et al. notes that the primary sensory and motor cortices are not included in either the task-positive or task-negative network because they are actually activated in both networks, and therefore show no intrinsic network preference. Therefore, these seemingly contradictory conclusions about the activation of the premotor and supplementary motor cortices seem to be a result of the fact that Mason et al. only examined when the brain was “at rest.” 

Using a self-report TUT method in addition to fMRI imaging during tasks varying in the degree of visual attention demanded, Andres-Hanna et al. (2010), attempted to distinguish default network activity resulting from the broadening of external attention compared to that associated with spontaneous cognitive processes (i.e., mind-wandering). As depicted in Figure A, significantly more task-unrelated thoughts were observed by the fMRI during the passive condition (with no event expectations) compared to either the broad or focal attention conditions, in which participants expected either peripheral or foveal events, respectively. From this finding the researchers concluded that default network activity is associated with spontaneous cognition, as those in the passive condition had an increased likelihood to engage in spontaneous cognition.

However, while Andrews-Hanna et al. examine mind-wandering by varying the degree of attention demanded (by manipulating subjects’ task expectations), thus affecting the likelihood of task-unrelated thoughts, the subjects’ conscious experience was not measured during scanning. Such reports were only recorded with a post-study questionnaire (Figure B).  Though these results do confirm the differences in spontaneous cognition observed during the fMRI scans, because they are reported after the scanning, the data could reflect default network activity resulting from factors other than mind-wandering.  Andrews-Hanna et al. therefore would have had more convincing findings had the self-reports been taken during scanning.

Stawarczyk et al. (2011), on the other hand, provide a better way to distinguish and therefore understand these task-unrelated thoughts than Andrews-Hanna (2010) by examining neural correlates of four different types of conscious experiences. They do so by categorizing thoughts by both task-relatedness and stimulus-dependency (See Figure 1) to examine if and how default network activation is associated with task-unrelated and stimulus-independent thought (i.e. mind-wandering). In doing so, they differentiate external distractions from mind-wandering thoughts, a distinction that has been poorly defined and at times completely neglected in the majority of past mind-wandering literature. By making such a distinction, we are able to better understand the default mode network and, more specifically, which of its regions correlate to different conscious experiences.

   

Stawarczyk et al. found that activation associated with task-unrelated and stimulus-independent thought (i.e. mind wandering) are both related to default mode network activity and that the effect can be additive in certain areas while other regions were found to be related to only one dimension. They suggest that, as opposed to being all or nothing, mind-wandering may be a gradual experience that is not necessarily mutually exclusive in relation to other conscious cognitive processes.  Compared to the other articles mentioned, the work by Stawarczyk et al. seems to provide the most valid approach to understanding the default network activity during mind-wandering as it uses thought-probes during fMRI image scans, therefore allowing for a more accurate depiction of the relationship between mind-wandering and associated default mode network activation.