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

Eelke Spaak eelke.spaak at donders.ru.nl
Thu Nov 21 18:27:43 CET 2013


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
>> _______________________________________________
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>> fieldtrip at donders.ru.nl
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>>
>
>
> --
> 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
>
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