Editorial: Through a Glass, Darkly: The Influence of the EEG Reference on Inference About Brain Function and Disorders

This Research Topic summarizes recent advances in a conflictive and controversy laden topic in neurophysiology: the selection of the electroencephalographic reference that is best for inference about brain function.Since its discovery, the human EEG has proven itself an indispensable tool for brain research.Despite this success story, there is a fundamental technical issue that has yet to be solve: selection of the correct EEG reference.Ideally one would like to measure neural activity restricted to certain brain regions.Since EEG amplifiers measure potential difference between the activities recorded by two electrodes, in addition to the active electrode, one must employ a reference electrode which should ideally be at zero.In theory, this might be achieved by placing the reference at a point infinitely far away.Yet the "infinite reference, " in practice, is an antenna for ambient noise which would pre-empt brain measurements-for example cephalic references that minimize unwanted signal pickup.Examples of such references are the unilateral-mastoid, ear, linked mastoids or ears, vertex, the tip of the nose, neck ring, etc.Unfortunately, all such references are doomed to fail since there is no point on the scalp or body surface where the potential is actually zero or a constant.This has serious consequences since the non-neutral reference may itself reflect physiological dynamic processes that will be inevitably embedded into all EEG recordings.Without solving the reference issue, we are looking at brain activity, as it were, "through a glass, darkly."This Research Topic focused the comparison of the effect that various EEG references may have on inference about brain function and disorders-with respect to both physical and computational issues.The crucial point is to determine the reference that best identifies neural activity and therefore be the basis of improved estimates of various linear and non-linear EEG features.These include spectra, amplitude, latency, coherence/correlation, network, symmetry/asymmetry, fractal dimension, complexity, covariance, and related statistical tests.If a single reference can be finally recognized universally as the optimal one for general use, we will have indeed rendered the "glass less opaque" and thus "know in part" more about brain function.Recent attempts to make this "glass" more transparent have been based on mathematically constructing a reference based on physical principles and subtracting it from all EEG recordings.