[Eeglablist] Re-referencing to Average Reference

[Tarik S Bel-Bahar]

in my opinion, average reference is a good bet when you have denser
coverage of the head with enough electrode density, something like 60 and
above,
preferably closer to 100 and above. But it also depends on what reference
was
active during recording, depending on the make of your nets.
For basic introductions you can
check out the links to various
resources for your education regarding re-referencing.

http://dionysus.psych.wisc.edu/lit/Topics/Psychophysiology/CacioppoText/Ch.2.pdf

   1. [PDF] from uni-konstanz.de
   2. http://www.springerlink.com/content/6785124141026861/fulltext.pdf
   3. http://sccn.ucsd.edu/pipermail/eeglablist/2008/002510.html
   4. [PDF] from tu-ilmenau.de
   5. http://www.springerlink.com/content/6785124141026861/
   6. http://onlinelibrary.wiley.com/doi/10.1111/1469-8986.3850847/abstract

*
*
*http://www.mrc-cbu.cam.ac.uk/research/eeg/eeg_intro.html*
Electric potentials are only defined with respect to a reference, i.e. an
arbitrarily chosen "zero level". The choice of the reference may differ
depending on the purpose of the recording. This is similar to measures of
height, where the zero level can be at sea level for the height of
mountains, or at ground level for the height of a building, for example.
For each EEG recording, a "reference electrode" has to be selected in
advance. Ideally, this electrode would be affected by global voltage
changes in the same manner as all the other electrodes, such that brain
unspecific activity is subtracted out by the referencing (e.g. slow voltage
shifts due to sweating). Also, the reference should not pick up signals
which are not intended to be recorded, like heart activity, which would be
"subtracted in" by the referencing. In most studies, a reference on the
head but at some distance from the other recording electrodes is chosen.
Such a reference can be the ear-lobes, the nose, or the mastoids (i.e. the
bone behind the ears). With multi-channel recordings (e.g. >32 channels),
it is common to compute the "average reference", i.e. to subtract the
average over all electrodes from each electrodes for each time point. This
distributes the "responsibility" over all electrodes, rather than assigning
it to only one of them. If a single reference electrode was used during the
recording, it is always possible to re-reference the data to any of the
recording electrodes (or combinations of them, like their average) at a
later stage of processing. In some cases "bipolar" recordings are carried
out, where electrode pairs are applied and referenced against each other
for each pair (e.g. left-right symmetrical electrodes).






On Sun, Jan 15, 2012 at 10:02 AM, Marco Montalto <montaltomarco at onvol.net>wrote:

> Dear all,
>
> I have collected data using a 32-channel EEG system. The electrodes were
> spread evenly on both hemispheres. These are the electrode locations:  FPz,
> FP1, FP2, Fz, F1, F2, F3, F4, FCz, FC1, FC2, Cz, C1, C2, C3, C4, C5, C6,
> T7, T8, Pz, P1, P2, P3, P4, P5, P6, PO7, PO8, Oz, O1 and O2. The electrodes
> were referenced to left earlobe. In EEGLAB I have re-refrenced the data to
> average reference. I have reviewed the literature and found conflicting
> opinions as to whether re-referecing to average reference when using 32
> channels is justifiable and defendable. Can anyone share his/her views
> regarding this matter? I am rather new to EEG so any help would be
> immensely appreciated. Thanks!
>
> Marco Montalto
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