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I am trying to better understand the relationship between the placement of EEG electrodes and the quality of the "brainwave" signals they produce.

There is the standard 10-20 system which has me wondering a few things:

  • Is the 10-20 system just a standard based on uniform distribution of the electrodes (so as to maximize brain signal collection)? Is it safe to assume one could increase the "density" of electrodes (again, in uniform distribution) and receive better quality signals? In other words, how does the placement & quantity of electrodes affect the quality of the information that can be obtain via EEG?
  • Is electrode gel strictly necessary? Is the gel used for adhesion, conductivity or both? I can't imagine it helps much with conductivity seeing the most of the scalp is (typically) covered with hair?
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  • The placement in terms of location on the scalp determines what region of the brain is sampled. The electrode density determines how many locations are sampled. The spatial resolution of an EEG has been reported to be 7 mm (Im et al., 2006), meaning the variability in determined source and verified source (via fMRI in the study cited) is approximately 7 mm. Hence, increasing the density below 7 mm may not be too helpful. The 10-20 system utilizes an inter-electrode distance of 60 - 65 mm (American Clinical Neurophysiology, 2006), indicating it may make sense to use more electrodes when high resolution is needed. Note, however, that MRI and MEG have better spatial resolution than EEG and are methods of choice when you wish to do accurate source localization. EEG has excellent properties in the temporal domain, and a combination of MEG/EEG has great characteristics in both spatial and temporal domains.

    Standardized EEG montages indeed promote uniformity so that different studies can be compared more efficiently.

  • Scrubbing with electrode paste primarily reduces impedance (increases conductivity). Hair and the skin don't conduct well and by applying a conductive paste impedances are substantially reduced. Mostly the technicians also scrape a little bit to ensure even better conductance by removing the fatty top layer of the skin. Also important is that differences in electrode impedances negatively impact recordings (American Clinical Neurophysiology, 2006). Impedance matching is hence necessary and using paste aids this purpose. So yes, conductive paste is essential.

Reference
- Im et al., J Neurosci Methods (2007); 161(1): 142–54

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  • $\begingroup$ Thanks for the awesome answer @Christiaan (+1) - the only part of your answer that I'm not quite following is the part surrounding spatial resolution. (a) When you state "The spatial resolution of an EEG is quite poor and has been reported to be 7mm..." what exactly do you mean (what exactly is "spatial resolution", and why is 7mm a poor value for it)? (b) It sounds like increasing density (e.g. 10-5 system) would help with this poor spatial resolution? Thanks again! $\endgroup$ – smeeb Feb 12 '16 at 10:54
  • $\begingroup$ @smeeb - I adapted the answer. $\endgroup$ – AliceD Feb 12 '16 at 11:42
  • $\begingroup$ I don't see how it follows from the resolution of EEG being X that there is no benefit to more narrow electrode spacing than X. For example, it is possible (and likely) that electrode spacing X is not sufficient to recover sources with accuracy X. After all, resolution refers to brain sources, spacing refers to sensor density. $\endgroup$ – user 49102 Feb 12 '16 at 14:37
  • $\begingroup$ If the error margin is 7 mm narrowing density down below that doesn't make much sense imo. $\endgroup$ – AliceD Feb 12 '16 at 15:36

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