Lead Field

[Gopakumar Venugopalan]

Greetings Padgaig, thank you for that explanation. I am analysing some
MEG/EEG data using EEGLAB (which incorporates several Fieldtrip
routines!). When I exported the data to SPSS-PC and graphs show values
in the 10,000 and 100,000 micro volt range. EEG data was gathered
together with the MEG in a MEG chamber. Am I seeing the same issue
there or is it something else. I appreciate any help.
regards
gopa


Quoting Pádraig Kitterick <p.kitterick at PSYCH.YORK.AC.UK>:

> Hi Cristiano,
>
> I'll attempt to help you with this but I'd also appreciate some input
> on
> this topic from others on the list to check my thinking is correct.
>
> I think your strange results are due to the mixing of several
> different
> units of measurement. If the positions of sensor and source are
> expressed in metres and the dipole moment in A-m (i.e. all in
> standard
> units), then the field strengths due to the source as calculated by
> compute_leadfield.m will be in Tesla. Of course, you can use cm or mm
>
> for your scale but that will just linearly scale the field values,
> i.e.
> converting the distances from metres to cm (increase of 10^2) will
> _decrease_ the field values (reduction of 10^2) if the same moment
> value
> is specified in both calculations. Similarly, expressing the moment
> in
> units of nA-m will also scale the field values linearly, but in that
>
> case it will increase the fields by 10^9 relative to the same moment
>
> expressed in A-m.
>
> For your example the use of cm will decrease the field values by 10^2
>
> and specifiying the dipole moment in nA-m will increase it by 10^9 -
> all
> this is relative to standard units. Therefore, if interpreted as
> Teslas,
> your fields are probably too large by a factor of 10^7 for your
> desired
> source stregth of 10nA-m. You could standardise your units by
> multiplying the resultant fields by 1e-7 or by using the following
> when
> you compute them:
>
> dip_mom = [1e-8 0 0]; % 10 nA-m as A-m
>
> but presumably as your sensor positions and orientations are in cm so
>
> you could either convert them and the dipole position to metres
> beforehand, or otherwise if you use the above moment value you will
> still have to multiply the resulting fields by 10^2 to compensate for
>
> the cm scaling, which should give you reasonable field strengths in
> Tesla.
>
> Hope that helps and does not confuse,
>
> Padraig
>
> Cristiano Micheli wrote:
> > Hi everybody
> > I tested the leadfield routine (compute_leadfield.m) for current
> dipole in
> > MEG forward solution.
> > How are the units expressed?
> > I expect the current dipole to be expressed in nA.m according to
> CTF
> > convention (i am using a 275 channels MEG CTF system), the
> gradiometers'
> > positions in cm and the lead field in Tesla.
> > Nevertheless i compared it with CTF software and there is quite a
> high
> > mismatch in the scaling factor, and the field is not perfectly
> distributed
> > as in CTF software forward solution.
> > In the Fieldtrip documentation it is mentioned a leadfield
> computation
> > method from Lütkenhöner, Habilschrift '92 which i could not find.
> How do i
> > get to the article?
> > I attach the code:
> >
> > dip_pos = [2 2 10]; % cm
> > dip_mom = [10 0 0]; % 10 nA*m
> > % grad: gradiometers structure
> > % vol:  conductive sphere model
> > lf = compute_leadfield(dip_pos, grad, vol,
> 'singlesphere','yes')*dip_mom';
> >
> > ----------------------------------
> > 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.