[Eeglablist] EEG questions

[Robert Oostenveld]

On 7 Mar 2004, at 22:36, Borna Noureddin wrote:

> I'm really sorry to post to this group, but I've been playing with 
> EEGLAB and - being new to EEG - have some basic dumb questions.  If 
> anyone could point me in the right direction, I'd appreciate it.
>
> - Is there any consensus on the spatial resolution of a typical scalp 
> electrode?  What is the smallest volume whose activity one can expect 
> to represent with scalp EEG?  How deep a structure can contribute?  
> I'm not asking about the resolution of any particular algorithm like 
> ICA, but the physical limits of scalp-recorded EEG.

It is not so simple just to define a "spatial resolution" with EEG as 
it is with MRI. Every EEG electrodes pick up anything, ranging from a 
1mm patch of cortex just underneath the electrode all the way up to 
global widespread activity. The EEG on any electrode is a mixture of 
all these activities. Source analysis tries to unmix the contributions 
of these activities based on a (spatial) source and head model, ICA 
tries to unmix them based on a statistical data model.

With high quality data containing only one signal that can be described 
with am equivalent dipole model and a very good volume conduction model 
you can in principle get a spatial resolution of a few mm (e.g. 
electrical SEP). If you have lower quality data, a less accurate head 
model, multiple spatially overlapping components, the resolution gets 
worse, all the way up to ~20cm (the size of the brain). Most 
importantly, the spatial resolution depends not on the EEG as 
measurement technique, but on the properties of the underlying brain 
activity.

> - Has there been any work done on the capacitive effects with scalp 
> EEG, both in terms of head models and electrodes?
>

Yes there has. See for example
* J.G. Stinstra, M.J. Peters. The volume conductor may act as a 
temporal filter on the ECG and EEG. Med Biol Eng Comput. 1998 
Nov;36(6):711-6.
* C. Gabriel, S. Gabriel, and E. Corthout. The dielectric properties of 
biological tissues: I. literature  survey. Phys Med Biol, 41:2231-2250, 
1996.
* S. Gabriel, R. W. Lau, and C. Gabriel. The dielectric properties of 
biological tissues: II. measurements in  the frequency range 10 Hz to 
20 GHz. Phys Med Biol, 41:2251-2269, 1996.
and references therein. For non-resistive electrode-skin interface 
effects, you would have to search into the older literature.

The consensus is that, in the frequency range of the scalp EEG, the 
capacitive effect of the tissues in the head are negligable. The 
unreliable descriptions of the geometry and of the condcutivity itself, 
and the anisotropy of the tissue, poses a much larger problem than the 
non-resistive (cappacitive and inductive) effects.

best regards,
Robert Oostenveld

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Robert Oostenveld, PhD
Center for Sensory-Motor Interaction (SMI)
Aalborg University, Denmark

and

F.C. Donders Centre for Cognitive Neuroimaging
University Nijmegen
P.O. Box 9101
NL-6500 AH Nijmegen
The Netherlands

Tel: +31 (0)24 3619695
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