[FieldTrip] LCMV giving conflicting results vs SAM and DICS

[Charidimos Tzagarakis]

Hi Don,
Thanks for your reply. I do agree that some of these beamformers
are probably better suited than others to study beta desynchronisation, or
generally questions in the frequency domain. I in fact started from DICS.
The reason I looked at LCMV and SAM is to be sure that I get consistent
results (and also because my experiment can also address some additional
time domain questions, so I wanted to see what happens when my data goes
from one to the other). I therefore tried to set them up so I that the
results from all 3 are similar. So you are correct in pointing out that my
question is why SAM and LCMV produce different results. Based on their
description within the fieldtrip website and mailing list (if I have
interpreted these correctly) they should be treating the covariance matrix
in the same way (to get a true evoked response covariance matrix you would
normally need the extra call to ft_timelockanalysis that I show at the last
piece of code in my post). Also , if the covariance matrix is indeed the
issue, it seems that the change in how it is treated in the
ft_sourceanalysis code (based on the "hack" I described) is unique to LCMV
(none of the other beamformer options share that piece of code) and is
active only when LCMV is given a precomputed filter which is (I think)
unusual.
Hence my question!
Best,
Haris

Charidimos [Haris] Tzagarakis MD, PhD, MRCPsych
University of Minnesota Dept of Neuroscience and Brain Sciences Center



On 23 October 2013 13:31, Rojas, Don <Don.Rojas at ucdenver.edu> wrote:

> Haris,
>
> Sorry - In my last post, I mistakenly put DICS and the Fieldtrip
> implementation of SAM into the same frequency domain category. In
> Fieldtrip, SAM is a time-domain technique and is not the same as the
> implementation of SAM that has been used in the published literature for
> beta ERD. So, are you then wondering why the two time-domain approaches
> produce differing results? That probably does depend on how the covariance
> matrix is calculated. Although I still think it is a bad idea to use a
> time-domain beamformer on motor beta ERD/ERS.
>
> Best,
>
> Don
>
>
>
> _______________________________________________
> fieldtrip mailing list
> fieldtrip at donders.ru.nl
> http://mailman.science.ru.nl/mailman/listinfo/fieldtrip
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://mailman.science.ru.nl/pipermail/fieldtrip/attachments/20131023/311bec3c/attachment-0002.html>