[FieldTrip] LCMV giving conflicting results vs SAM and DICS

[Eelke Spaak]

Hi Charidimos,

Thanks for your elaborate e-mail. We discussed this issue in
yesterday's FieldTrip meeting. It is very reasonable that you expect
the three methods to produce the same results, and the code/interface
indeed suggests that this would be the case. In other words, we need
to clean up this part of the code to produce consistent results;
however, this might take a while.

We believe the discrepancy is caused by the fact that you compute the
covariance while specifying cfg.keeptrials = 'yes', which is
unnecessary while computing the filter. This results in a covariance
matrix with dimensions trial X channel X channel, which then is
subsequently averaged over trials during ft_sourceanalysis. Likely the
filter is still in fact computed on the averaged covariance, as it
should be. However, the subsequent projection of data through the
filter will not first average the covariance, and will in essence only
project the first trial through the filter. (This is because the dip{}
fields will be struct arrays of dimension nTrial X 1, and in an
assignment a(1).b = 2; a(2).b = 3; x = a; then x will be 2, so
everything but the first element of a struct array is ignored in an
assignment statement.)

Could you try to compute the covariance with cfg.keeptrials = 'no' and
then run LCMV again? This will still compute the covariance on the raw
traces, not on the average. If our hunch is right, you should get the
correct results then.

As said, this is indeed messy and unclear, our apologies for that.

Finally, could you tell us what part of the code you commented out to
make the results consistent? We suspect it will be between lines 790
and 835, but it would be helpful to know for sure.

Best,
Eelke


On 23 October 2013 21:43, Charidimos Tzagarakis <haristz at gmail.com> wrote:
> 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
>>
>>
>>
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>
>
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