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Default Mode Network (DMN) activity, measured with fMRI, typically increases during internally directed thought and decreases during external tasks. However, Crittenden et al. (2015) and Smith et al. (2018) found increased DMN activity during demanding external task-switches between different cognitive domains, compared to within-domain switches and task repeats. In our previous experiment, we aimed to replicate this effect in a similar paradigm and test if DMN switch activity reflects increased representation of broader context. In Core DMN, we found significant activity for all task switches (versus task repeats), and stronger activity for switches between rest and task. Although scene context was decodable, there was no evidence that this differed by switch type. Surprisingly, we did not see greater DMN activity at between-domain versus within-domain switches. This may depend on overall task-set complexity, because we used two domains and four task types whereas the previous studies used three domains and six task types. We hypothesize that the disappearance of the domain-dependent task-switch effect in DMN is due to the decrease of number of domains in the paradigm. We aim to investigate this in a follow-up study contrasting DMN activity during task-switch trials when the tasks are drawn from either two or four domains. Furthermore, in the same paradigm we plan to compare DMN activity for task-switches between domains belonging to different cognitive chunks formed through learning and practice. We hypothesize that DMN activity will be greater for between-chunk than within-chunk switches. To assess whether effects might be due to changes in task expectancy for different numbers of domains, half of the experiment will use the same task expectancy as previous experiments, where the trials are balanced by switch type and globally by task, while including all task transitions; the other half of the experiment will restrict the possible task transitions to create an even task expectancy regardless of switch type and number of domains.

Recent discoveries of Default Mode Network (DMN) activity during large external task switches and task restart suggest that the role of the network is not yet fully addressed by theories strictly linked to internally directed or self-related cogntive models. Crittenden et al (Crittenden et al., 2015) and Smith et al (Smith et al., 2018) found that DMN activity responds to external tasks and is strong both when cognitive focus is relaxed and when a new task model is established. Both authors suggested a possible context-encoding role of the DMN, whereby the waxing and waning of DMN activity might represent the broader cognitive context, which might gain focus during salient task transitions as well as at rest. The conceptualisation of context representation in the DMN has also been suggested in previous literature, linking DMN activity to a ‘situation model’ (Ranganath, 2012) or a representation of broad features such as spatial surroundings and time, which tend to be temporally stable and characterise extended “episodes” within which transient task events are embedded. We propose to further investigate whether DMN activity encodes information about the broader context, or episode, and whether this representation is strengthened during reconstruction of cognitive focus during task switches. By using functional magnetic resonance imaging, we aim to scan participants while they perform a task-switch paradigm at fixation, with background scene images representing broader contexts. The context representation theory for DMN would be supported if we see faster recognition of context changes, and more encoding of background context in DMN subnetworks, during large task switches.

Additional file 1: Supplementary table. Subgroup analysis for patients with HIV.