1 Functional interactions between large - scale networks during memory search Running Head : Large - scale networks supporting memory search

Neuroimaging studies have identified two major large-scale brain networks, the default mode network (DMN) and the dorsal attention network (DAN), which are engaged for internally and externally directed cognitive tasks respectively, and which show anticorrelated activity during cognitively demanding tests and at rest. We identified these brain networks using independent component analysis (ICA) of functional magnetic resonance imaging (fMRI) data, and examined their interactions during the free-recall task, a self-initiated memory search task in which retrieval is performed in the absence of external cues. Despite the internally directed nature of the task, the DAN showed transient engagement in the seconds leading up to successful retrieval. ICA revealed a fractionation of the DMN into three components. A posteromedial network increased engagement during memory search, while the two others showed suppressed activity during memory search. Cooperative interactions between this posteromedial network, a right-lateralized frontoparietal control network, and a medial prefrontal network were maintained during memory search. The DAN demonstrated heterogeneous task-dependent shifts in functional coupling with various subnetworks within the DMN. This functional reorganization suggests a broader role of the DAN in the absence of externally directed cognition, and highlights the contribution of the posteromedial network to episodic retrieval. 3 In studies of the human memory system, when a specific external cue successfully prompts memory retrieval, functional magnetic resonance imaging (fMRI) techniques reliably identify a core network of brain regions spanning the hippocampal formation, posterior cingulate cortex, posterior parietal cortex, and medial prefrontal cortex These regions belong to a brain network known as the default mode network (DMN). They show spontaneous correlation during resting states (Raichle et al. 2001), and exhibit increased activity during self-referential tasks (Buckner and Carroll 2007; Andrews-Hanna et al. 2010b; Qin and Northoff 2011), suggesting that these regions play a critical role in internally directed cognitive processes. In contrast to the DMN, Corbetta and Shulman (Corbetta and Shulman 2002) described the dorsal attention network (DAN), which is implicated in attentional orienting to external stimuli, and consists of dorsal parietal cortex including the intraparietal sulcus, and dorsal prefrontal cortex, including the putative frontal eye fields. These two networks are consistently anticorrelated during cognitive tasks requiring external attention (Fox et al. 2005). A study by Sestieri et al. (Sestieri et al. 2011) illustrates this anticorrelation across tasks: While a perceptual search task engaged the DAN and suppressed the DMN, an episodic retrieval task engaged the DMN and suppressed the DAN. …