Lead Field

[jan-mathijs schoffelen]

Hi guys,

May I quickly add to the discussion that I remember having seen that  
Cristiano uses myhead.hdm as an input to prepare_headmodel, and the  
resulting vol-structure for compute_leadfield.
Is myhead.hdm a multisphere model? Just to be sure: The call to  
compute_leadfield with additional inputs 'singlesphere' 'yes' does  
not lead to a single-sphere-based leadfield, as far as I know.
Single sphericity is only ensured when vol contains just one .o, and  
one .r.

Yours,

JM

On Jun 23, 2008, at 4:16 PM, Pádraig Kitterick wrote:

> The method implemented in meg_leadfield1.m gives the same result as  
> the equations of Sarvas. It's just a different formulation of the  
> forward problem. I've produced identical leadfields calculated with  
> several different packages, including fieldtrip, and also a direct  
> implementation of the Sarvas equations.
>
> It is possible that the CTF software is accounting for the coil  
> loop radii and other machine-specific inputs to the calculations,  
> but these rarely change the fields by large amounts. Are you using  
> the same sphere to approximate the head in both instances? The  
> meg_leadfield1.m code assumes that the centre of the single sphere  
> is at the origin, (0,0,0). If the centre of the sphere isn't at the  
> origin, then you need to adjust the dipole and sensor locations by  
> subtracting the centre of the sphere from their coordinates. The  
> CTF system could well be doing this adjustment using a pre- 
> programmed sphere centre based on either a general estimation or  
> coregistration information it has available to it. Unless you can  
> access all the information that it is using to do the calculations,  
> it will be very hard to get identical answers.
>
> The best thing is to ensure that you have a good estimate of the  
> centre of the single sphere for your sensor array based on an  
> average head/brain or a particular subject you are trying to model.  
> Then your fields will be as accurate, despite not being identical  
> in absolute numerical values to those produced by the CTF software.
>
> Padraig
>
> Cristiano Micheli wrote:
>> Hi Pádraig
>> Thank you for the answer. It helped to fix the scaling factor. Now  
>> i get a
>> magnetic field of order of ~1 mT for a 10nA.m dipole ~3 cm far  
>> from the pick
>> up coils (with Fieldtrip routine).
>> Still i cannot get the same field as for the CTF software. In CTF  
>> leadfield i get [min max] values of: 1e-06 *[ -67.1 100.4] Tesla (275
>> channels), while in Fieldtrip i get: [-0.0011 0.0016] Tesla.
>> I think the two routines use different equations. CTF implements  
>> Sarvas'
>> forward model for a spherical conductor in a homogeneus medium  
>> ("Basic
>> mathematical and electromagnetic concepts of the biomagnetic inverse
>> problem", Sarvas J, 1987).
>> I cannot get to the source of Fieldtrip's method for the forward  
>> model,
>> which is mentioned in meg_leadfield1.m routine head comment  
>> (adapted from
>> Luetkenhoener, Habilschrift '92). It sounds like an habilitation  
>> work.
>> I found from Lütkenhöner :
>> "Möglichkeiten und Grenzen der neuromagnetischen Quellenanalyse",  
>> Münster:
>> LIT-Verlag, 1992
>> Unfortunately i do not have access neither to the book nor to  
>> formulas.
>> How can i check it? If it does not depend on implementation  
>> details (which sounds also plausible
>> since the error is quite high), what can be again the problem to  
>> make the
>> two fields match?
>> Thank you
>> Cristiano
>> ----------------------------------
>> 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/ 
>> fcdonders/fieldtrip.
>
> -- 
> Pádraig Kitterick
> Graduate Student
> Department of Psychology
> University of York
> Heslington
> York YO10 5DD
> UK
>
> Tel: +44 (0) 1904 43 3170
> Email: p.kitterick at psych.york.ac.uk
>
> ----------------------------------
> 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/fcdonders/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/fcdonders/fieldtrip.