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Effective EEG Connectivity Analysis of Episodic Memory Retrieval

Effective EEG Connectivity Analysis of Episodic Memory Retrieval Chung-Yeon Lee (cylee@bi.snu.ac.kr) Computer Science and Engineering, Seoul National University Seoul 151-744, Republic of Korea Byoung-Tak Zhang (btzhang@snu.ac.kr) Computer Science and Engineering & Cognitive Science and Brain Science Programs, Seoul National University Seoul 151-744, Republic of Korea Abstract Episodic memory formation is associated with large-scale neuronal activity distributed across the cortex. Decades of neuroimaging and patient lesion studies demonstrated the correlation between the roles of specific brain structures in episodic memory retrieval. Distributed, coordinated and synchronized activities across brain regions have also been investigated. However, neuronal mechanisms based on effective connectivity underlying the coordination of this anatomically distributed information processing into introspectively coherent cognition have remained largely unknown. Here we investigate the information flow network of the human brain during episodic memory retrieval. We have estimated local oscillation amplitudes and asymmetric inter-areal synchronization from EEG recordings in individual cortical anatomy by using source reconstruction techniques and effective connectivity methods during episodic memory retrieval. The strength and spectro-anatomical patterns of these inter-areal interactions in sub-second time-scales reveal that the episodic memory retrieval involves the increase of information flow and densely interconnected networks between the prefrontal cortex, the medial temporal lobe, and some subregions of the parietal cortex. In this network, interestingly, the SFG acted as a hub, globally interconnected across broad brain regions. Keywords: Episodic memory retrieval; Information flow, Effective connectivity; EEG, Memory retrieval network. Introduction The next step in the progression of neuroscience is building neuro-cognitive models that describe the dynamics and interaction patterns of brain regions on a macroscopic scale. Indeed, recent advances in cognitive neuroscience have focused on the role of inter-areal interactions between various specialized brain regions and functional connectivity in human cognition (Stevens, 2009; Yarkoni et al., 2010, Park & Friston, 2013). The study of human brain connectivity generally falls under three categories: structural, functional, and effective connectivity (Bullmore & Sporns, 2009). Structural connectivity refers to the static anatomical structure of the brain. This can be studied in vivo using invasive axonal labeling techniques, magnetic resonance imaging (MRI, Lauterbur, 1973) or diffusion tensor imaging (Moseley et al., 1990) devices. Both functional and effective connectivity are defined with respect to a cognitive task and denote synchronized activity of two neuroanatomical regions during task execution. However, effective connectivity is able to deal with asymmetric or causal dependencies between the two regions, while functional connectivity has only a symmetrical nature. Thus, the term “information flow” is often used to indicate directionally specific effective connectivity between two brain structures. Dynamic causal modeling (DCM), structural equation modeling (SEM), transfer entropy, and Granger-causal methods are popular effective connectivity methods and they can be applied to functional MRI (fMRI) and/or electrophysiological imaging data such as electroencephalography (EEG), intracranial EEG (iEEG), and magnetoencephalography (MEG). Episodic memory formation is a complex neurocognitive process that is associated with large-scale neuronal activity distributed across the cortex. However, neuronal mechanisms based on the effective connectivity underlying the coordination of this anatomically distributed processing have remained largely unknown. Decades of neuroimaging and patient lesion studies correlated the roles of specific brain regions to episodic memory retrieval such as the prefrontal cortex (Blumenfeld & Ranganath, 2007; Duarte et al., 2005), the medial temporal lobe (Eichenbaum et al., 2007; Mitchell & Johnson, 2009; Squire et al., 2004; Simons & Spiers, 2003; Vargha- Kadem et al., 1997) and some subregions of the parietal cortex (Hutchinson et al., 2009; Spaniol et al., 2009; Vilberg & Rugg, 2008). Furthermore, distributed and coordinated activities across brain regions are regarded to be important to the memory retrieval processes (Buzsaki, 1996; Eichenbaum, 2000; McClelland et al., 1995; Nadel & Moscovitch, 1997; Norman & O'Reilly, 2003; Teyler & DiScenna, 1986). Synchronized activity in the local field potential (LFP) is also related to coordinating these processes (Fell & Axmacher, 2011). Recently, following these previous findings, frequency multiplexing of brain regions involved in episodic memory retrieval (Watrous et al., 2013) have also been studied, but effective connectivity has not yet been considered. Here in this paper, we are primarily concerned with the information flow network of the human brain during