[FieldTrip] ft_sourcedescriptives

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

Hi Matt,

I am not sure what you’re after - because your question is not fully well-posed - but I think that the actual question is about how the ‘pow’ field is computed, and how this differs between the plain output from ft_sourceanalysis (when using cfg.method = ‘mne’), and ft_sourcedescriptives (when using as input the output of ft_sourceanalysis, and using cfg.projectmom = ‘yes’).

After ft_sourceanalysis (with cfg.method = ‘mne’) you get a source-structure with source.avg.pow. The pow is the magnitude (squared) of the dipole moment over time, obtained by squaring and summing across the three cardinal x/y/z orientations.

After ft_sourcedescriptives (with cfg.method = ‘mom’, which only works if previously ft_sourceanalysis has been called with cfg.mne.projectmom = ‘yes’) the pow-field of the output contains the magnitude squared dipole moment along the principal axis of the dipole’s activity (i.e. estimated by means of an svd).

So, in short, the difference between the 2 ‘pow’s is that the first one assumes a model where the dipole is rotating over time, whereas the second one models the dipole’s activity under the assumption that it has a fixed orientation over time.


Best wishes,
Jan-Mathijs



On 3 Jul 2021, at 00:27, Matt Moradi via fieldtrip <fieldtrip at science.ru.nl<mailto:fieldtrip at science.ru.nl>> wrote:

Hi Experts,

What is the main difference between output of ft_sourceanalysis and the ft_sourcedescriptive? we are using them with MNE method to get the source of the difference between two ERPs and we cannot quite figure out if we are feeding the correct parameters ...
at the end of this page at:
https://www.fieldtriptoolbox.org/tutorial/minimumnormestimate/<https://urldefense.com/v3/__https://www.fieldtriptoolbox.org/tutorial/minimumnormestimate/__;!!HJOPV4FYYWzcc1jazlU!v0m9C0651gDPW_WeBCkmGdDM0U5aX_3rC3P7iMEkPzbEp5Y3IbwOSyXQjl3nwwM2Zh5aczYrSJz-rUs$>

They are plotting the difference between the two ERPs  from output of ft_sourcedescritive with the projectmom option, but doing the same subtraction (sdDIFF.avg.pow = sdFIC.avg.pow - sdFC.avg.pow)
with the avg.pow output from ft_sourceanalysis gives a different solution.
Appreciate any thoughts!
Thanks,
Matt





--
Matt
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