The organization of physiological brain networks

Abstract

One of the central questions in neuroscience is how communication in the brain is organized under normal conditions and how this architecture breaks down in neurological disease. It has become clear that simple activation studies are no longer sufficient. There is an urgent need to understand the brain as a complex structural and functional network. Interest in brain network studies has increased strongly with the advent of modern network theory and increasingly powerful investigative techniques such as “high-density EEG”, MEG, functional and structural MRI. Modern network studies of the brain have demonstrated that healthy brains self-organize towards so-called “small-world networks” characterized by a combination of dense local connectivity and critical long-distance connections. In addition, normal brain networks display hierarchical modularity, and a connectivity backbone that consists of interconnected hub nodes. This complex architecture is believed to arise under genetic control and to underlie cognition and intelligence. Optimal brain network organization becomes disrupted in neurological disease in characteristic ways. This review gives an overview of modern network theory and its applications to healthy brain function and neurological disease, in particular using techniques from clinical neurophysiology, such as EEG and MEG.

Introduction

The brain can be seen as a complex anatomical and functional network. Theories that apply to complex networks in general, such as modern network theory and complex systems theory, are increasingly used in neuroscience to understand how normal brain function arises and what happens in disease. They provide a totally new perspective leading to some very new insights. This review gives an overview of the fundamentals and applications of modern network theory in the study of normal and developing brain function. In addition, we aim to explain how it is possible that various brain diseases can be understood in terms of failing network characteristics. The focus will be mainly on functional aspects of brain networks, but structural connectivity, animal studies, and modeling will also be discussed to provide a wider perspective. Since network studies are based upon a fundament of functional connectivity, the neurophysiological basis of normal and disrupted synchronization is discussed first. Subsequently, the basic concepts and tools of modern network theory are introduced. The application of network theory is illustrated by studies in healthy subjects, and various disorders such as schizophrenia, depression, dementia, trauma and epilepsy with an emphasis on functional, and in particular neurophysiological approaches. Finally we suggest a general framework for the study of the organization of physiological brain networks.