In search of the Rosetta Stone for scalp EEG: Converging on reference-free techniques

Almost a hundred years have passed since Hans Berger’s historic discovery that electrical brain waves can be recorded from the human scalp (Berger, 1929). Since that time, the electroencephalogram (EEG) has been recognized as a real-time, noninvasive measure of both tonic (e.g., at rest or during sleep) and phasic neuronal activity (e.g., as evoked responses to physical or cognitive events). Many approaches have been developed to identify, separate, quantify, and compare the temporal and spectral properties of the EEG, as evidenced in the pages of this journal over the past 60 years. The EEG remains a valuable and cost-effective tool for a wide range of clinical and basic research purposes, regardless of the recent numerous developments of complementary neuroimaging measures. In addition to an unparalleled temporal resolution, important technological advances, such as dense electrode arrays with over a hundred channels that allow an evenly-spaced scalp coverage, offer dramatically increased topographic capacities in a recording montage with improved data quality and reduced preparation time, owing to high impedance amplifiers and miniature preamplifiers located inside the scalp sensor. However, despite the impressive advances and continued promise of these methods, we still lack an universal key to decipher the functional meaning of the scalp-recorded EEG. One well-known problem in particular arises again and again, and often in forms that may be unrecognized at first: because an EEG signal must be quantified as a potential difference between any two sites, thereby yielding relative rather than absolute measures, the properties of the reference, whether determined by its physical location or its computational characteristics, will have a fundamental impact on the signal of interest. For example, if two sites are equipotential, no EEG activity is observed between them, no matter what the absolute potential may be. Another implication is that the information provided by a difference measure is unaffected by its direction, apart from its arbitrary sign (i.e., the selection of one of a pair as reference is inherently arbitrary).