[FieldTrip] Help with units of measurements with ft_sourceanalysis

[Schoffelen, J.M. (Jan Mathijs)]

Hi Sara,

If you want to change the units of the data, you can use ft_convert_units for that. I am not sure how smooth it will work in your case, and it should be noted that the prerequisite for this function to work is that the units are specified in the data. In your case, this probably means that you should have a field data.chanunit, or data.elec.chanunit or so, which specifies the units of the electrophysiological signal.

Also note, that a covariance is a quadratic measure, and that the units are in V^2 (or so), which results in doubling of the exponent if powers of 10 are involved (e.g. 10^-6 in amplitude of trial/avg results in 10^-12 in values for the covariance). To be on the safe side, I recommend to convert the units early in the pipeline, before calling ft_timelockanalysis instead of afterwards.

Good luck,

Jan-Mathijs


> On 22 Oct 2024, at 15:31, Sara Cinelli via fieldtrip <fieldtrip at science.ru.nl> wrote:
>
> Hello everyone,
> I have a question that is creating me some issues. I would like to ask you about the unit of measurements I have to use in the ft_sourceanalysis function. I already know that the data in input for ft_compute_leadfield (sourcemodel,headmodel and elec) has to be in the standard unit,i.e. meters (m) and volt (V), in order to obtain the measure in V/A*m.
> My question is the following: my TEP measure is microV and the elec unit is mm. Do I have to change the measures in V (and in m), so multiply the avg*10^-6? The TEP obtained from ft_timelockanalysis has differents measure inside, in particular the covariance. Do I have to change these other measures as well? If so, how? Thank you very much!
>
> Sara
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