[FieldTrip] Merging runs with different grad.tra matrix

[Marisa Monika Amalie Birk]

 Dear FieldTrippers,

I am working with MEG data from multiple runs, each ca. 8 min long and
saved as separate files. I have applied MaxFilter with a target head
position and preprocessed each run individually, including ICA, and now
want to concatenate the data before doing source reconstruction.

Up until the ICA, the grad struct was identical between runs. However,
using ft_componentanalysis and ft_rejectcomponent changed the grad.tra
matrix, leading to differences across runs. This means I can't simply merge
the datasets. This tutorial
<https://www.fieldtriptoolbox.org/faq/how_are_electrodes_magnetometers_or_gradiometers_described/>
emphasizes how important the grad.tra field is for computing the leadfields
and that it should remain consistent with the data manipulations - so I
assume that I can not simply use the grad.tra matrix from before the ICA.

Do you have suggestions on how I should proceed? Should I perform source
reconstruction separately for each run, or is it better to concatenate the
runs first and then do the ICA on the full dataset? The runs are also
rank-deficient and have slightly different ranks due to MaxFiltering, so I
am unsure about merging them before ICA to avoid rank mismatches.

Are there alternative approaches you would recommend?

Thank you!

Best regards
Marisa


*Marisa Birk*
PhD Student in Cognitive and Brain Sciences,
Action Recognition and Concepts Group (CIMeC)
<https://www.cimec.unitn.it/en/929/action-recognition-and-concepts-group>

Center for Mind and Brain Sciences (CIMeC),
University of Trento, Italy
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