[biomag] paper announcement

Michael Wibral wibral at BIC.UNI-FRANKFURT.DE
Fri Aug 21 10:46:38 CEST 2009 

Dear Tom, dear Sara,

I think there is possibly a slight misunderstanding between the two of you with respect to the 'arbitrary baseline' of the axial gradiometers. As far as I understand the issue, this baseline is arbitrary and individual per sensor, but relatively stable and can be removed by 'calibration' before the measurement (i.e. in a null measurement without brain-derived magnetic fields), i.e. in a setup step of the machine.  All brain-derived magnetic fields are a difference to this null-state and should be correct in their field distribution as they were added after the calibration step. So there is no need to subtract the experimental pre-stimulus baseline while measuring and thereby to sacrifice information about brain status in the experimental baseline.

Tom, would you please correct me by a response to the list if I am wrong on the machine related/technical issues here.

Thanks,
Michael


> -----Ursprüngliche Nachricht-----
> Von: "Sara GONZALEZ ANDINO" <Sara.GonzalezAndino at HCUGE.CH>
> Gesendet: 21.08.09 10:20:22
> An: FIELDTRIP at NIC.SURFNET.NL
> Betreff: Re: [FIELDTRIP] [biomag] paper announcement


> 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.
> 



----------------------------------
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.
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