[FieldTrip] Motor beta activity - DICS solution more noisy than sensor data?

[Stan van Pelt]

Hi Eelke, Have you tried comparing the results with a time-domain beamformer? You could e.g. try LCMV on 15-25Hz-bandpassed data. This should give you in theory similar results. Best, Stan ----- Oorspronkelijk bericht -----
> Van: "Eelke Spaak" <eelke.spaak at donders.ru.nl>
> Aan: "FieldTrip discussion list" <fieldtrip at science.ru.nl>
> Verzonden: Donderdag 21 november 2013 18:27:43
> Onderwerp: Re: [FieldTrip] Motor beta activity - DICS solution more
> noisy than sensor data?
> Thanks for the input Vitória. Are you using Fourier input to
> sourceanalysis, or 'powandcsd'? I'm wondering whether the csd
> computation inside sourceanalysis might be to blame. Will investigate
> this tomorrow :)
> Eelke
> On Nov 21, 2013 6:01 PM, "Vitoria Piai" < v.piai.research at gmail.com >
> wrote:
> > Hi all, Eelke,
> > Maybe adding to Eelke's remark, although I'm not working with motor
> > beta activity and lateralised index.
> > I'm also using DICS and my very clear, focal sensor-level effects
> > all
> > of a sudden become localised to the whole brain or a whole
> > hemisphere!
> > I happen to have heard from Jan-Mathijs that there may something
> > going
> > on at the moment, but at least for now, Eelke, I don't think this
> > error is unique to our data (and I was actually going to post my
> > incompatible sensor-source results here soon, so now it's less work
> > for me :)
> > Looking forward to hearing your updates,
> > (I'm interrupting my source-level analyses for the time to get a
> > better feeling for the sensor-level data first)
> > Vitória
> > On 21-11-2013 17:36, Eelke Spaak wrote:
> > > Fellow FieldTrippers,
> > > Currently I am looking at a contrast for left- versus right-hand
> > > index
> > > finger button presses. As expected, on sensor level (combined
> > > planar
> > > gradient, grand average) I see a clear lateralisation in beta band
> > > power starting at least 0.5s before the button press (see
> > > https://db.tt/Rtch3Qjy ). Both 'blobs' are significant; there is
> > > clearly more beta power ipsilateral to the response hand. I would
> > > prefer to do further analyses on source level, so I attempt to
> > > reconstruct the sources for this effect using DICS beamformer
> > > (common
> > > filter, applied to both conditions separately; fixedori and
> > > realfilter
> > > = 'yes'). The grand average results for this (again contrast left
> > > vs
> > > right response hand) are shown at https://db.tt/IBQZG0d8 . (Ignore
> > > the
> > > R/L-flip, this is radiological convention.)
> > > As you can see, the source level solution is much more blurry than
> > > on
> > > sensor level. This picture is without using any regularisation
> > > (lambda
> > > parameter), the results are even worse when I use lambda = '5%'.
> > > The
> > > negative blob (right hand higher power than left) becomes
> > > 'marginally
> > > significant' on source level (p ~ 0.06) where it was p < 0.001 on
> > > sensor level. The positive blob is nowhere near significant. Also,
> > > the
> > > individual results are much less topographically consistent on
> > > source
> > > than on sensor level (explaining the worse statistics).
> > > I have checked the segmentation of my MRIs, the 'gray' seems to be
> > > nicely within the head all the time. Also, I have manually
> > > verified
> > > the alignment of headmodel, sourcemodel, and gradiometer
> > > information
> > > for all subjects.
> > > As a final note, the above sensor-level plot was taken from a
> > > 'slice'
> > > out of a planar-gradient time-frequency analysis (mtmconvol). The
> > > ingredient for the beamformer was an mtmfft fourier spectrum on
> > > the
> > > axial gradiometer data, obtained for just the time-frequency range
> > > of
> > > interest (subselect toilim [-0.5 0], mtmfft foi = 23, tapsmofrq =
> > > 7).
> > > When I compute condition-averaged power based on these fourier
> > > spectra
> > > and look at the contrast, the results are again as expected:
> > > https://db.tt/n2P3UKcQ (of course less localised because of axial
> > > gradient vs planar). The freq structures underlying this contrast
> > > are
> > > exactly the same as those going into ft_sourceanalysis, so the
> > > problem
> > > must be in the source analysis step (and/or in the preparation of
> > > the
> > > geometric information, although these seem fine by visual
> > > inspection).
> > > Does anyone have any idea that might explain these seemingly
> > > contradictory results? I would have expected demixing to improve
> > > signal-to-noise ratio, rather than worsen it.
> > > Thanks!
> > > Best,
> > > Eelke
> > > ______________________________ _________________
> > > fieldtrip mailing list
> > > fieldtrip at donders.ru.nl
> > > http://mailman.science.ru.nl/ mailman/listinfo/fieldtrip
> > --
> > Vitória Piai
> > PhD Candidate
> > Donders Institute for Brain, Cognition and Behaviour
> > Centre for Cognition, Radboud University Nijmegen
> > Montessorilaan 3, B.01.05
> > 6525 HR Nijmegen
> > The Netherlands
> > Email : V.piai at donders.ru.nl
> > Phone : +31 24 3612635
> > www.vitoriapiai.com
> > ______________________________ _________________
> > fieldtrip mailing list
> > fieldtrip at donders.ru.nl
> > http://mailman.science.ru.nl/ mailman/listinfo/fieldtrip
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-- Stan van Pelt, PhD Donders Institute for Brain, Cognition and Behaviour Centre for Cognition Montessorilaan 3, B.01.19 6525 HR Nijmegen tel: 024-3616288
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