title: Impact of the reference choice on scalp EEG connectivity estimation.
abstract: Several scalp EEG functional connectivity studies, mostly clinical, seem to overlook the reference electrode impact. The subsequent interpretation of brain connectivity is thus often biased by the choice of a non-neutral reference. This study aims at systematically investigating these effects.
  
  
  
  As EEG reference, we examined the vertex electrode (Cz), the digitally linked mastoids (DLM), the average reference (AVE), and the reference electrode standardization technique (REST). As a connectivity metric, we used the imaginary part of the coherency. We tested simulated and real data (eyes-open resting state) by evaluating the influence of electrode density, the effect of head model accuracy in the REST transformation, and the impact on the characterization of the topology of functional networks from graph analysis.
  
  
  
  Simulations demonstrated that REST significantly reduced the distortion of connectivity patterns when compared to AVE, Cz, and DLM references. Moreover, the availability of high-density EEG systems and an accurate knowledge of the head model are crucial elements to improve REST performance, with the individual realistic head model being preferable to the standard realistic head model. For real data, a systematic change of the spatial pattern of functional connectivity depending on the chosen reference was also observed. The distortion of connectivity patterns was larger for the Cz reference, and progressively decreased when using the DLM, the AVE, and the REST. Strikingly, we also showed that network attributes derived from graph analysis, i.e. node degree and local efficiency, are significantly influenced by the EEG reference choice.
  
  
  
  Overall, this study highlights that significant differences arise in scalp EEG functional connectivity and graph network properties, in dependence on the chosen reference. We hope that our study will convey the message that caution should be used when interpreting and comparing results obtained from different laboratories using different reference schemes.
authors.affiliation: Department of Neuroscience, Imaging and Clinical Sciences, 'G. d'Annunzio' University of Chieti-Pescara, Chieti, Italy. Institute for Advanced Biomedical Technologies, 'G. d'Annunzio' University of Chieti-Pescara, Chieti, Italy.
authors.forename: Federico
authors.initials: F
authors.lastname: Chella
authors.forename: Vittorio
authors.initials: V
authors.lastname: Pizzella
authors.forename: Filippo
authors.initials: F
authors.lastname: Zappasodi
authors.forename: Laura
authors.initials: L
authors.lastname: Marzetti
mesh_terms: D000328:Adult; D000465:Algorithms; D003198:Computer Simulation; D004566:Electrodes; D004569:Electroencephalography / Q000706:statistics & numerical data*; D005260:Female; D006257:Head; D064368:Healthy Volunteers; D006801:Humans; D007092:Imagination / Q000502:physiology; D008297:Male; D008416:Mastoid / Q000502:physiology; D008959:Models, Neurological; D009415:Nerve Net / Q000502:physiology; D012015:Reference Standards; D015203:Reproducibility of Results; D012535:Scalp; D055815:Young Adult