[FieldTrip] FT_SCALPCURRENTDENSITY options and elec units

Johanna Zumer johanna.zumer at donders.ru.nl
Wed Sep 5 12:41:38 CEST 2012

Hi Akiko,

Hopefully someone else can answer your main SCD question (although
ft_scalpcurrentdensity provides the citations for the original papers on
which these 2 methods are based).

But for now, just a quick answer to your units question.  The best way to
convert the units of a FieldTrip data structure is with ft_convert_units,
for example:  elec_out = ft_convert_units(elec_in, 'm').     If elec.unit
does not already exist in the structure, then either you can set it first
if you know the units, or FT will do its best to assume which units the
structure is in.

Also note that the 'help' of ft_scalpcurrentdensity states that "the
spatial distribution still will be correct" even if units do not match,
which should explain why the topoplot shows the same results.


2012/8/31 Akiko Ikkai <akiko.ikkai at gmail.com>

> Hi,
> I'm trying to run plv analysis, first acquiring SCD of my data with
> ft_scalcurrentdensity, and am confused about the difference between
> cfg.method = 'finite' and 'spline.'  Both gave similar results
> short-range, but 'spline' gives higher plv at long range plv. I'd love it
> if someone could explain or suggest reading on the difference between these
> two methods (or is one method preferred over the other in some cases?).
> Also, I'm not sure whether or not I should convert my elec.pnt  values,
> since the code ft_scalcurrentdensity says that
> "Note that the skin conductivity, electrode dimensions and the potential all
> have to be expressed in the same SI units." My elec.pnt are in cm, and
> I'm using the default conductivity (0.33 S/m). Do I need to do something
> like elec.pnt = elec.pnt/100 to convert the unit from cm to m? With or
> without conversion, I got the same results (viewed with ft_topoplotTFR),
> and I'm a little confused... Thank you in advance for your time! Akiko
> --
> Akiko Ikkai, Ph.D.
> Postdoctoral Fellow
> Department of Psychological and Brain Sciences
> Johns Hopkins University
> Ames Hall, 3400 N. Charles St.
> Baltimore, MD 21218
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> fieldtrip at donders.ru.nl
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