[Eeglablist] Defining channels locations for Biosemi data

[Makoto Miyakoshi]

Dear Enrico,

There are differences between spherical head model and MNI head model. The
former is perfectly round (like a soccer ball) while the latter is more
like a human head. The electrode montages optimized to these models have
different coordinates.

> (electrodes defined by BESA trepass the head model border)

This means that the electrodes are below certain line of circumference. You
can roughly estimate this line to be the one that connects Iz, Nz, and both
Pre-Auricular locations.

Just FYI, there was a related discussion probably 3-4 years ago about what
exactly the scalp topo outline means. I investigated it and found that at
least for EEGLAB's scalp topos the definition of the line leaves ambiguity;
it's not exactly 50% of something. I think I read Oostenveld & Praamstra
2001 or something, and also communicated with the first author of the paper
(we know him of course!)

Makoto



On Fri, Nov 18, 2016 at 8:57 AM, Fucci, Enrico <e.fucci.12 at alumni.ucl.ac.uk>
wrote:

> Dear all,
>
>
> I hope you will help me elucidating this simple question:
>
>
> After importing EEG data (64 electrodes cap) from Biosemi format into
> EEGlab (through Biosig), I procede to define channel coordinates. For this,
> I tried two different ways, that result in slightly different electrode
> configurations when plotted on a 2D template.
>
>
> The first method consists in adding a *.loc file *where electrode
> locations are derived from the template "GSN64v2_0.sfp" provided by EEGlab.
> Regarding the second method, I just automatically retrieve channel
> locations using the BESA file "standard-10-5-cap385.elp" implemented in
> dipfit, after having renamed channel labels from Biosemi to 10:20
> convention.
>
>
> Both methods give locations coherent with the Biosemi layout, but slightly
> different in terms of matching with the 2D template (electrodes defined by
> BESA trepass the head model border). Does anyone know what is this
> difference caused by?
>
>
> You can find the .loc file and the two plots on the 2D template at the
> following link (do not mind the external electrodes) :
>
>
> https://www.dropbox.com/sh/56x8z3u8j80lyg9/AAC2lm95zy4eKxOr1kHSByMFa?dl=0
>
>
> Thank you very much for the support.
>
>
> Best,
>
>
> Enrico Fucci
>
>
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-- 
Makoto Miyakoshi
Swartz Center for Computational Neuroscience
Institute for Neural Computation, University of California San Diego
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