Functional and Effective Connectivity: A Review

Address correspondence to: Karl J. Friston, The Wellcome Trust Centre for Neuroimaging, Institute of Neurology, 12 Queen Square, London WC1N 3BG, United Kingdom, E-mail: [email protected]

Introduction

This review of functional and effective connectivity in imaging neuroscience tries to reflect the increasing interest and pace of development in this field. When discussing the nature of this piece with Brain Connectivity's editors, I got the impression that Dr. Biswal anticipated a scholarly review of the fundamental issues of connectivity in brain imaging. On the other hand, Dr. Pawela wanted something slightly more controversial and engaging, in the sense that it would incite discussion among its readers. I reassured Chris that if I wrote candidly about the background and current issues in connectivity research, there would be more than sufficient controversy to keep him happy. I have therefore applied myself earnestly to writing a polemic and self-referential commentary on the development and practice of connectivity analyses in neuroimaging.

This review comprises three sections. The first represents a brief history of functional integration in the brain, with a special focus on the distinction between functional and effective connectivity. The second section addresses more pragmatic issues. It pursues the difference between functional and effective connectivity, and tries to clarify the relationships among various analytic approaches in light of their characterization. In the third section, we look at recent advances in the modeling of both experimental and endogenous network activity. To illustrate the power of these approaches thematically, this section focuses on processing hierarchies and the necessary distinction between forward and backward connections. This section concludes by considering recent advances in network discovery and the application of these advances in the setting of hierarchical brain architectures.

The Fundaments of Connectivity

Here, we will establish the key dichotomies, or axes, that frame the analysis of brain connectivity in both a practical and a conceptual sense. The first distinction we consider is between functional segregation and integration. This distinction has a deep history, which has guided much of brain mapping over the past two decades. A great deal of brain mapping is concerned with functional segregation and the localization of function. However, last year the annual increase...