Functional connectivity in the brain: effects of anesthesia.

Functional connectivity has been defined as "the temporal correlation of a neurophysiological index measured in different brain areas." Since its definition, functional connectivity analysis has been used to describe temporal correlations across multiple spatial scales in PET imaging, single-unit and local field potential recordings, electroencephalography (EEG) and magnetoencephalography (MEG), optical imaging, and fMRI. These findings have been used to identify coactivating brain regions as functional networks. In some instances, as in the case of the default mode network (DMN), functional connectivity has been used to describe "modes" of brain function. The opportunity to probe the anesthetized state using functional connectivity analysis has given rise to a diverse literature over the past two decades. The examination of functional connectivity in the anesthetized state is of relevance to both anesthesiologists and neuroscientists, as it has the potential to elucidate still unclear mechanisms of anesthesia while offering insight into intrinsic functional activity in the brain. Complications have arisen, however, in the form of a lack of standardization of anesthetics, dosages, depths of anesthesia, and methods of functional connectivity analysis across studies. The present work attempts to examine, elucidate, and integrate the insight that functional connectivity analysis of the anesthetized state has generated thus far.