[Eeglablist] ERP analyses and average referencing

[Thomas Ferree]

Yvonne,
Average referencing will not remove muscle artifact.  You should either
delete stretches of data with lots of muscle, or try to filter using ICA or
some
other method.

To answer your question about average reference, the conservative answer
is 64 electrodes.  See the paper by Srinivasan R et al. (1998).  That is if
you
want the average reference to be interpretable as the voltages relative to
infinity.  See the paper by Ferree T (2006).  In practice 32 (or 26)
electrodes
may give approximately the same result, but it is pushing the limits.

That said, you can compute the average reference for much fewer
electrodes, but you can't interpret that as voltages relative to infinity.
Rather the interpretation is limited to say that the average reference
is not explicitly dependent on any single electrode as reference, and
you are measuring relative to the ' spatial average' in some loose
statistical sense.

A final note, if you used linked-mastoid reference for acquisition,
in which the two mastoids were literally physically connected together
with a near zero-impedance wire path between them, then you can
not transform to average reference in any rigorous way.  That setup
forces the potential at the two mastoid to be nearly equal, which is
surely not the case in the head without electrodes attached, and
that artifact is not eliminated by average reference.  See discussion
in the text by Nunez and Srinivasan (2006).

-- 
Thomas Ferree, PhD
Department of Radiology
UT Southwestern Medical Center
Email: tom.ferree at gmail.com
Voice: (214) 648-9767

On Tue, Sep 16, 2008 at 9:20 PM, Yvonne Tran <Yvonne.Tran at uts.edu.au> wrote:

> Dear All
>
> We are currently working with spinal cord injured participants and have
> recorded some oddball data. We have been using A1 and A2 mastoid for
> reference channels, however, with this particular group we are experiencing
> increased muscle tension in this region (which cannot be prevented, as some
> participants are unaware that they are tensing up), and therefore when the
> data are re-referenced the other EEG channels become flooded with muscle
> tension noise. This can be overcome when we re-reference using average
> referencing. My question is how many electrodes (evenly distributed around
> the scalp) will be ok for average referencing for ERP analyses? We have 26
> EEG channels.
>
> Any suggestions/opinions appreciated!
>
> Thank you
> regards
> Yvonne
>
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://sccn.ucsd.edu/pipermail/eeglablist/attachments/20080921/e0a85ca9/attachment.html>