Investigating default mode network activation during external task switches and movie watching

Abstract

The Default Mode Network (DMN), typically associated with internally-directed thought, shows increased activity during demanding external task switches between cognitive domains, as reported by Crittenden et al. (2015) and Smith et al. (2018), which challenges prevailing views of DMN function. What might the role of DMN be in response to external cognitive transitions? To address this question, Chapter 2 explores whether DMN activity during task switches reflects increased representation of broader context, specifically the background scene behind the focal task. Significant activity was observed in the Core DMN for all task switches compared to task repeats, with stronger activation for switches between rest and task. Although the background scene was attended, recalled, and neurally decodable, this did not vary by switch type. Therefore, external task switches activated the DMN without enhanced processing of the background context. Chapter 3 further investigates the factors that modulate the DMN at switches. The complexity and structure of the task set appear to play a critical role. This study examined whether the DMN’s response to task switches depends on the complexity of the task set, manipulated by the number of tasks or by abstract groupings based on instructional order. Core DMN activation at task switches was not affected by the number of tasks but depended on the instructional order of task groups. In Chapter 4, I explore the DMN’s role in representing and updating a mental model through its recruitment during various transitions in naturalistic stimuli, such as movies. The findings indicate that DMN subnetworks are sensitive to objective transitions in stimuli and subjective event boundaries, supporting the DMN’s role in interpreting continuous external events and detecting changes in the contextual model. Finally, Chapter 5 discusses DMN involvement in various external cognitive transitions, future directions for research, and the implications in clinical and developmental contexts.