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

[Eelke Spaak]

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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>>
>
>
> --
> 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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