[FieldTrip] Query about source/gridpoint size after ft_prepare_sourcemodel

Ravi Mill rdm146 at newark.rutgers.edu
Tue Jul 24 01:45:43 CEST 2018


Dear Fieldtrippers


I've been scouring the FT documentation and mailing list but can't seem to find the answer to a fairly simple question (which seems to have remained unanswered previously https://mailman.science.ru.nl/pipermail/fieldtrip/2016-August/010816.html): how does one know the size/'radius' of the individual source gridpoints modeled by Fieldtrip?


I've been warping MNI template source models (with RO selected from a functional atlas of ~300 regions) to individual subject MRI headmodels to extract source timeseries via LCMV beamformer. In ft_prepare_sourcemodel there are seemingly options to control the *spacing* between gridpoints in a whole-brain source grid (cfg.grid.resolution), but nothing to control how large the individual sources are (either for whole-brain grids, or a priori cfg.grid.pos values that I've been using).


Does this mean that the size of individual gridpoints cannot be changed within ft_prepare_sourcemodel? Are they also in absolute terms fairly small (e.g. ~1mm)? If so, is the only way to extract 'larger' sources to fit a whole-brain grid (e.g. using the 4mm-spaced source template provided by FT), identify gridpoints after ft_sourceanalysis that fall within a specified Euclidean distance (e.g. 5mm) of the functional atlas MNI coordinates, and then e.g. average over all those gridpoints? I'm concerned that larger source gridpoints might have larger SNR...


Thanks in advance!

Ravi


--
Ravi Mill, PhD
Post-doctoral researcher
Center for Molecular and Behavioral Neuroscience (CMBN)
Rutgers University

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