Interplay Between Default-Mode and Task-Positive Networks: Functional Characterization of the Brain's Large-Scale Neural Systems.

Abstract

Recent work has demonstrated that the human brain is functionally organized into distinct large-scale networks, with particular attention focused on the default-mode network (DMN) and the anti-correlated “task-positive” networks. Despite the growing evidence that these neural systems are intrinsically connected at rest and during the performance of cognitive functions, little is known about network relationships during tasks that actively recruit DMN, such as social cognition. Characterizing how the functions and the interactions of the DMN may modulate brain activity in other large-scale neural systems may be a critical step in advancing our understanding of network dynamics. Using behavioral and functional magnetic resonance imaging (fMRI) experiments, this dissertation aims: 1) To characterize network dynamics when functions carried out by the DMN are required, and 2) to determine the modulatory effects of task demand on network dynamics in processing these functions. Four experiments were developed to address these aims. Using a task that probes a fundamental aspect of social cognition - appraising another individual, experiment 1 showed parallel recruitment of the DMN (medial prefrontal and posterior cingulate cortices) and the task-positive network (pre-SMA, dACC, bilateral fronto-parietal cortices). Connectivity analyses (psychophysiological interaction) further showed functional interaction within the DMN, and with the task-positive network, both vary as a function of social preference. In another set of experiments, a novel dual-task paradigm was developed that parametrically manipulated factors known to affect cortical activity in the default-mode and task-positive networks: social cognition and spatial working memory demand, respectively. Two behavioral experiments showed selective interference, manifested as a drop in working memory accuracy, between spatial working memory and the evaluative appraisal of self, suggesting functional overlap. Finally, a neuroimaging experiment adopted this dual-task paradigm to examine the interactions between DMN, social cognition and task demand. Significant social cognition-by-task demand interactions were present in multiple regions of the DMN (medial prefrontal regions) and the task-positive networks (primarily posterior parietal foci). Overall, these results suggest that network dynamics, at least between the two neural systems considered herein, are dependent on social cognition as well as task demand.