Elsevier

NeuroImage

Volume 19, Issue 2, June 2003, Pages 466-470
NeuroImage

Regular article
The elusive concept of brain connectivity

https://doi.org/10.1016/S1053-8119(03)00112-5Get rights and content

Abstract

Neurons and neural populations do not function as islands onto themselves. Rather, they interact with other such elements through their afferent and efferent connections in an orchestrated manner so as to enable different sensorimotor and cognitive tasks to be performed. The concept of functional connectivity and the allied notion of effective connectivity were introduced to designate the functional strengths of such interactions. Functional neuroimaging methods, especially PET and fMRI, have been used extensively to evaluate the functional connectivity between different brain regions. After providing a brief historical review of these notions of brain connectivity, I argue that the conceptual formulations of functional and effective connectivity are far from clear. Specifically, the terms functional and effective connectivity are applied to quantities computed on types of functional imaging data (e.g., PET, fMRI, EEG) that vary in spatial, temporal, and other features, using different definitions (even for data of the same modality) and employing different computational algorithms. Until it is understood what each definition means in terms of an underlying neural substrate, comparisons of functional and/or effective connectivity across studies may appear inconsistent and should be performed with great caution.

Section snippets

Functional and effective connectivity—a very brief history

To frame the issues that make defining brain connectivity so difficult, a brief historical review is necessary. Investigations of the brain during the last century have been marked by a peculiar dichotomy: whereas many neuroanatomists worked diligently to ascertain how different neuronal populations were connected to one another so as to form networks,2

Definitional difficulties

As can be seen from this brief historical overview, numerous investigators over the years have claimed that they were examining something akin to functional or effective connectivity. However, the multiple types of data that are used to evaluate functional interactivity differ from one another in many features, including spatial and temporal resolution, and whether the data represent neuron activities, neural ensemble activities, or activities (electrical or hemodynamic) of macroscopic brain

Acknowledgements

I want to thank Karl Friston and Rolf Kotter for an extremely interesting round of discussions on the issue of connectivity.

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