The default mode network is not as simple as it seemed. There are probably several configurations. A recent paper (by D Stawarczyk and others) has looked at the difference between the default network when the subject is not attending to a task and when the subject is ignoring sensory stimulating from the outside world.
Here is the abstract:
The default mode network (DMN) is a set of brain regions that consistently shows higher activity at rest compared to tasks requiring sustained focused attention toward externally presented stimuli. The cognitive processes that the DMN possibly underlies remain a matter of debate. It has alternately been proposed that DMN activity reflects unfocused attention toward external stimuli or the occurrence of internally generated thoughts. The present study aimed at clarifying this issue by investigating the neural correlates of the various kinds of conscious experiences that can occur during task performance.
Four classes of conscious experiences (i.e., being fully focused on the task, distractions by irrelevant sensations/perceptions, interfering thoughts related to the appraisal of the task, and mind-wandering) that varied along two dimensions (task- relatedness and stimulus-dependency) were sampled using thought-probes while the participants performed a go/no-go task. Analyses performed on the intervals preceding each probe according to the reported subjective experience revealed that both dimensions are relevant to explain activity in several regions of the DMN, namely the medial prefrontal cortex, posterior cingulate cortex/precuneus, and posterior inferior parietal lobe. Notably, an additive effect of the two dimensions was demonstrated for midline DMN regions. On the other hand, lateral temporal regions (also part of the DMN) were specifically related to stimulus-independent reports. These results suggest that midline DMN regions underlie cognitive processes that are active during both internal thoughts and external unfocused attention. They also strengthen the view that the DMN can be fractionated into different subcomponents and reveal the necessity to consider both the stimulus- dependent and the task-related dimensions of conscious experiences when studying the possible functional roles of the DMN.
The digits between 1 and 9 were shown at the center of a screen. Subjects were asked to be as quick and accurate as possible in responding to each number except if the number was 3. Series of stimuli were followed by a thought-probe which interrupted the task. For each probe, subjects were asked to characterize the conscious experience they has in the few trials prior to the probe. They were given four possible responses: on-task, task-related interferences, external distractions, mind-wandering. In total the responses were respectively 32, 22, 26, 21%. Subjects had training trials, and trials in and out of a scanner. This is an interesting blend of high-tech fMRI scanning, cognitive computer screen and keyboard experimentation and reporting of conscious thoughts.
I think it may be too early to label the default network as having one, two of even four or five functions. I assume that there are various network configurations for various tasks and likewise various configurations for various ‘resting’ or default conditions. The worry-wart that is stewing over some imagined problem is likely to have a very different mind-wandering configuration to the person anticipating their up-coming vacation. I look forward to examinations of the default network for years to come.
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) DOI: 10.1371/journal.pone.0016997