Gopakumar Venugopalan venug001 at BAMA.UA.EDU
Fri Jun 20 21:01:12 CEST 2008

```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,
>
>
> Cristiano Micheli wrote:
> > Hi everybody
> 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
> > 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
> > % vol:  conductive sphere model
> '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
> http://listserv.surfnet.nl/archives/fieldtrip.html and
> http://www.ru.nl/fcdonders/fieldtrip.
> >
> >
> >
>
> --
> Pádraig Kitterick
> 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