Segregation and Integration

Anatomical and functional segregation refers to the existence of specialized neurons and brain areas, organized into distinct neuronal populations grouped together to form segregated cortical areas (Shipp and Zeki, 1985; Zeki, 1993). The concept of anatomical segregation is rooted in the notion that specific brain processes or functions can be localized to specific anatomical regions of the human brain, an idea that is central to the history of neurology and cognitive neuroscience (Phillips et al., 1984). Maps of cortical regions, such as those assembled by Ungerleider and Mishkin (1982), Van Essen and Maunsell (1983), Zeki and Shipp (1988), and Felleman and Van Essen (1991) have provided increasingly refined network diagrams of multiple anatomically and functionally distinct areas of the primate visual cortex. These specialized and segregated brain regions contain neurons that selectively responded to specific input features (such as orientation, spatial frequency, or wavelength), or conjunctions of features (such as faces). Segregated areas maintain distinct patterns of connections with other areas, which are instrumental in defining these specialized local response properties (Passingham et al., 2002). Segregation can be found even within single cortical regions, where functionally distinct populations of neurons often remain spatially segregated. At least some intraregional (Gilbert and Wiesel, 1989; Tanigawa et al. 2005) and interregional (Angelucci et al., 2002) connections linking such populations are found to be patchy or clustered, preserving segregation.

Anatomical segregation entails that important correlates of specific functional brain states are found in localized changes of neuronal activity within specialized populations. However, segregated and specialized brain regions and neuronal populations must interact to generate functional dynamics. Coherent perceptual and cognitive states require the coordinated activation, i.e. the functional integration, of very large numbers of neurons within the distributed system of the cerebral cortex (Bressler, 1995; Friston, 2002). Electrophysio-logical studies have shown that perceptual or cognitive states are associated with specific and highly dynamic (short-lasting) patterns of temporal correlations (functional connectivity) between different regions of the thalamocortical system. Bressler has carried out numerous studies examining task-dependent large-scale networks of phase synchronization in primate and human cortex (Liang et al., 2000; Bressler and Kelso, 2001; Brovelli et al., 2004). Patterns of inter-regional cross-correlations have been found to accompany the performance of specific cognitive tasks in cats (e.g. Roelfsema et al., 1997), primates (Bressler, 1995) and humans (e.g. Srinivasan et al., 1999; Von Stein et al., 1999; Varela et al., 2001; Munk et al., 2002). Mcintosh has documented changes in brain functional connectivity related to awareness (Mcintosh et al., 1999), and most recently through recording differential interactivity of the human medial temporal lobe with other regions of the neocortex (Mcintosh et al., 2003). Human neuroimaging experiments have revealed that virtually all perceptual or cognitive tasks, e.g. object recognition, memory encoding and retrieval, reading, working memory, attentional processing, motor planning and awareness are the result of activity within large-scale and distributed brain networks (Mcintosh et al., 1999; 2000).

Common to most theoretical frameworks dealing with network aspects of cognition is the idea that integration across widely distributed brain regions requires neuronal interactions along inter-regional pathways. in the cortex, such interactions are mediated by the extensive and massive network of cortico-cortical connections. When these structural substrates of integration are disabled or disrupted, resulting in the disconnection of neuronal populations, specific functional deficits are often observed. While many observations suggest that disruptions of structural connections can result in deleterious effects on functional brain dynamics, we still lack a principled understanding of how structural connections determine dynamics. In the brain, as in most other biological systems, structure and function are strongly interdependent -however, a comprehensive theoretical framework describing their interrelationship in the networks of the cerebral cortex remains elusive. In the remainder of this chapter, we focus on a set of measures that quantify structural connections and functional dynamics and we review several computational and empirical approaches that, utilizing such measures, aim at uncovering structural determinants of functional brain dynamics.

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