[Eeglablist] Dipoles and Volume Conduction

[Makoto Miyakoshi]

Dear Ahmed,

> My question is, why the dipoles distribution is symmetric over the scalp?

I'm not an expert of this field, but the answer to 'why dipole?' should be
found in electromagnetics. I hope someone who knows this topic help us here.

> According to my understanding there is an infinite number of dipoles may
contribute in a specific source.  Is there any justification (Please
correct me if I’m wrong)?

In order for scalp channel to observe meaningful EEG, certain amount of
neurons that are aligned and activating together are necessary. That's like
a prerequisite for EEG measurement.

>Also why the volume conduction effect (transmission of electric current
from primary source electrode to other electrodes) can be seen in some
experiments only while it’s much less in some other experiment?

That's wrong. It's always there.

> What's the experimental factors may effect on volume conduction?

It must be very very very stable and can be hardly affected by any
experimental factors.

Makoto

On Wed, Dec 3, 2014 at 2:14 AM, Ahmed Almurshedi <ahmed19875000 at yahoo.com>
wrote:

> Dear Experts,
>
> I have noticed in some ERP experiments that the response is flipped in the
> posterior part than anterior part of scalp. For example in checkerboard
> pattern VEP, P100 response on mid occipital electrode and N100 response on
> the frontal electrode.
> According to the discussion with colleagues they said that ERP response
> are usually from the generation of a dipole, the field of which is picked
> up at different electrode locations.  Thus, some will pick up the
> positive end, and others will pick up the negative end. This is often
> called phase reversal, as the phase of waveform is 180 degrees reversed.
>
> My question is, why the dipoles distribution is symmetric over the scalp?
> According to my understanding there is an infinite number of dipoles may
> contribute in a specific source.  Is there any justification (Please
> correct me if I’m wrong)?
>
> Also why the volume conduction effect (transmission of electric current
> from primary source electrode to other electrodes) can be seen in some
> experiments only while it’s much less in some other experiment? What's the
> experimental factors may effect on volume conduction?
>
>  Thank you,
>
> AHMED
>
>
>
>
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-- 
Makoto Miyakoshi
Swartz Center for Computational Neuroscience
Institute for Neural Computation, University of California San Diego
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