mmiyakoshi at ucsd.edu
Tue Oct 7 13:34:24 PDT 2014
If you care this issue, I recommend you think it in this way: choosing a
channel and/or reference electrode to study e.g. frontal lobe activity *is
to use a spatial filter*. The problem of this (default) spatial filter is
volume conductance and scalp mixing, and it is clear that you can never
address these issues even if you re-reference to other channels. This is
why we use ICA as it provides the more reasonable spatial filter.
On Sat, Oct 4, 2014 at 12:51 PM, Nicholas Rosseinsky <
rosseinsky.nicholas.m at gmail.com> wrote:
> Oops - sorry: hit send waaaaaay too early ...
> 1. Eric asked about references concerning re-referencing.
> Here are some, not specifically concerned with IC decomposition though:
> Nunez, P. L. (2010). REST: a good idea but not the gold standard. *Clinical
> neurophysiology: official journal of the International Federation of
> Clinical Neurophysiology*, *121*(12), 2177
> Qin, Y., Xu, P., & Yao, D. (2010). A comparative study of different
> references for EEG default mode network: the use of the infinity reference. *Clinical
> neurophysiology*, *121*(12), 1981-1991.
> Hu, S., Stead, M., Dai, Q., & Worrell, G. A. (2010). On the recording
> reference contribution to EEG correlation, phase synchorony, and coherence. *Systems,
> Man, and Cybernetics, Part B: Cybernetics, IEEE Transactions on*, *40*(5),
> 2. I may well be wrong, but it seems to me that the reference question
> could be discussed a little more, generally. Notably, the average reference
> seems to be though of as "harmless" or "default", but obviously it
> subtracts from the data the average fluctuation of brain activity relative
> to e.g. mastoid potential! In an idealized case in which e.g. mastoid
> electrodes are already "at" some idealized "nominal ground" (Nunez 2010),
> this means that average referencing is subtracting out the common
> electrophysiologically-relevant activity captured in
> channel-potentials-relative-to-mastoid. (I'm aware of the unrealizability
> of this idealized case, and of the problems concerned with the potential of
> the body-as-battery ... I'm just sayin').
> Notably, try *visualizing* (making a topoplot movie of) alpha-band
> continuous EEG before and after average referencing. There are
> stereotypical and large spatiotemporal travelling patterns present in
> non-averaged data that disappear after averaging (disclaimer: *in the
> data I am currently looking at,* which has significant quality and
> scalp-coverage limitations). IC decomposition of non-averaged data
> certainly seems to have a spatially "ugly" component that captures e.g.
> common alpha activity - but to throw this out because it doesn't "look
> dipolar" is surely the wrong criterion. The question is whether this kind
> of activity is relevant to analyses. If alpha travelling waves affect task
> performance (Patten et al., 2012), and if you are interested in analysing
> or identifying such effects ... take care with choosing average reference.
> As I understand it (please, educate me, listers!) average reference is
> theoretically innocuous only if electrodes completely cover the spherical
> head; the further away one is from this (again, practically-unrealizable)
> ideal, the more *potentially* "impactful" average-referencing *might* be
> . And in other cases (my data may be particularly poor in this regard),
> it's going to be a judgment call best guided by:
> 1. visualizing your data before and after preprocessing to know what
> effects your pre-processing steps are having; and,
> 2. making an intelligent, context-dependent, analysis of how any
> preprocessing effects are going to affect your subsequent analyses.
> I don't think there are general principles here, and:* that doesn't mean
> that the choice doesn't matter, *i.e. "no general principles" doesn't
> mean "choosing average reference won't affect my analyses".
> Patten, T. M., Rennie, C. J., Robinson, P. A., & Gong, P. (2012). Human
> cortical traveling waves: dynamical properties and correlations with
> responses.*PloS one*, *7*(6), e38392.
> Hope that helps some, and please, if musings under 2 are nonsense - help
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Swartz Center for Computational Neuroscience
Institute for Neural Computation, University of California San Diego
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