Weak Task Synchronization of Default Mode Network in Task Based Paradigms

Default mode network (DMN) are the areas of the brain that get deactivated upon the presentation of externally focused, attention demanding tasks. Interestingly, these areas are consistently deactivated irrespective of the nature of the task. Recent evidence shows that the behavior of the DMN may be more nuanced than what was earlier believed. We wanted to investigate the dynamics of DMN further and utilized novel statistical approaches to study the same. We analysed four publicly available datasets (total 223 subjects) across seven tasks related to different cognitive modalities and found out that there is a large amount of variability in the presumed task-negative network across subjects, trials and regions. We designed a method called Inter Trial Temporal Synchronization Analysis (IT-TSA) and Inter Subject TSA (IS-TSA) to analyse variability across trials and subjects respectively. We found out that all task-negative regions and even weak task-positive regions have low synchronization across trials and subjects. We hypothesize that the DMN variability might be more due to neural differences rather than structural or HRF variations. We also investigated the use of non GLM technique like the Inter Subject Correlation (ISC) and found that it may only be suitable to study tightly synchronized task-positive regions. Our study challenges the understanding of DMN as a task-negative region and advances the findings that DMN acts as an active component associated with a unique set of self-referential, autobiographical processes which are deactivated differentially and non linearly across trials and subjects in the presence of extrinsic processes.