[biomag] paper announcement

Fri Aug 21 10:14:45 CEST 2009

Dear Dr. Tom,
thanks for your interest in our paper 

 "EEG/MEG Source Imaging: Methods, Challenges, and Open Issues"
http://www.hindawi.com/journals/cin/2009/656092.html 
and for bringing to discussion such important topics.

I do not know if all the authors of the manuscript receive mailings
from  these two lists. Thus, although  not necessarily on behalf of all
the authors, I would like to make some comments about your points.

>	I disagree. Radial gradiometers, as used in CTF MEG systems,
have an arbitrary baseline that MUST be removed.

We thought it was obvious that an arbitrary baseline need to be removed
from measurements. Now, if your system really has an arbitrary (i.e.
unknown and independent for each sensor) base lines, then such system
cannot yield true magnetic field distributions. I guess this is not the
case. Our comment was actually a warning about the facts that 1)
modifying the base line does change the map (i.e the field distribution)
and therefore the estimated source for both EEG or MEG. 2) Significant
physiological effects (e.g. pre attentive states) are also part of the
"baseline" and will be removed by this procedure.   

>	You're talking EEG only again. Stop saying EEG/MEG when you only
know about EEG. Adding sensors to an MEG system _increases_ spatial
resolution.

Hopefully the mathematics and physics behind EEG and MEG are the same,
then: 

1) If measurement M(r) ( for EEG and MEG !) are to be continuos then it
is mathematically necessary that closeness between two sensors placed at
r1 and r2 implies closeness of the measurements M(r1) and M(r2). From
this derives that adding sensors cannot indefinitely increase spatial
resolution, and  that numerical ill-conditioning emerge as a consequence
of the linear dependence of measurements (i.e. lead field rows). Since
noise also grows with the amount of sensors, there is a limit beyond
which no new information is added in the measurements but just noise.

2) If you prefer the physics to the mathematics you can look at 
Geselowitz equation describing the connection (In a realistic head
model) between the magnetic field at/ near the scalp and  the electrical
potential at the interfaces. While this is not a demonstration it might
help you to understand "de que va la cosa" i.e  how  things work.     

3) You might be also interested in reading Malmivuo's papers (quoted in
the manuscript you refer)) comparing spatial resolution for both EEG and
MEG. In agreement with previous point (2) no significant differences in
spatial resolution seems to exist between these two modalities.

4) Similar conclusions were obtained using different analysis
procedures in "Spatial resolution of neuronal generators based on EEG
and MEG measurements". International Journal of Neuroscience 68: 93-105,
1993 (by Pascual Marqui and Biscay Lirio).

In summary I think that our comments are valid for both EEG and MEG. 

regards

rolando
www.electrical-neuroimaging.ch 

-- 
Dr. Tom
---
I would dance and be merry,
Life would be a ding-a-derry,
If I only had a brain.
        -- The Scarecrow

>>> "Tom Holroyd (NIH/NIMH) [E]" <tomh at KURAGE.NIMH.NIH.GOV> 20.08.2009
21:16 >>>
Rolando GRAVE wrote:
> Dear Colleagues,
> 
> We would like to call your attention to the recent publication:
> http://www.hindawi.com/journals/cin/2009/656092.html 
> 
> EEG/MEG Source Imaging: Methods, Challenges, and Open Issues
> 
> by:
> Katrina Wendel, Outi Väisänen, Jaakko Malmivuo, Nevzat G. Gencer,
Bart
> Vanrumste, Piotr Durka, Ratko Magjarević, Selma Supek, Mihail Lucian
> Pascu, Hugues Fontenelle and Rolando Grave de Peralta Menendez
> 

>>From the paper:

2.2. Things to Avoid

...

(1) Baseline correction. Varying the values of individual electrodes
either by “arbitrary” baseline shifting or by scaling factors
changes the surface maps and thus the estimated sources. Although linear
inverse solutions are rather stable (continuity with respect to the
data), the application of base line correction to two conditions (that
will be compared on the basis of their sources) can produce artificial
differences induced by the correction and not by the real sources. 

	I disagree. Radial gradiometers, as used in CTF MEG systems,
have an arbitrary baseline that MUST be removed.

(3) The use of very high density of sensors might also jeopardize the
source analysis due to different kinds of noise at different sensors.
Moreover, no significant information is added after approximately 128
electrodes due to the noise levels. Lastly, some sensors might measure
more artifacts than others due to their location near active muscles.

	You're talking EEG only again. Stop saying EEG/MEG when you only
know about EEG. Adding sensors to an
	MEG system _increases_ spatial resolution.

-- 
Dr. Tom
---
I would dance and be merry,
Life would be a ding-a-derry,
If I only had a brain.
        -- The Scarecrow

----------------------------------
The aim of this list is to facilitate the discussion between users of
the FieldTrip  toolbox, to share experiences and to discuss new ideas
for MEG and EEG analysis. See also
http://listserv.surfnet.nl/archives/fieldtrip.html and
http://www.ru.nl/neuroimaging/fieldtrip.

----------------------------------
The aim of this list is to facilitate the discussion between users of the FieldTrip  toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/neuroimaging/fieldtrip.