[FieldTrip] Fwd: ASSR and midline activity

Mon Dec 19 22:44:31 CET 2016

Thank you, Marc!
That was exactly my point - how to differentiate "genuine" ASSR source from
"artificial"...
What other localization method is worth to try?
As for sensor-level data - as you can see in the magnetic field
distribution - the "frontal/midline" cluster might be the results of the
"meeting" of two magnetic field produced by dipoles in the auditory
cortex...
Best Regards,
Olga.

On Mon, Dec 19, 2016 at 3:29 PM, Marc Recasens <recasensmarc at gmail.com>
wrote:

> Hi Olga
>
> I haven't looked at your code in detail. Just wondering whether you have
> considered the possibility to be picking up genuine ASSR source activity in
> frontal areas?
> Additional ASSR generators outside auditory cortices have been described
> previously using both MEG (https://www.ncbi.nlm.nih.gov/pubmed/15925187)
> and PET (https://www.ncbi.nlm.nih.gov/pubmed/15276678).
> It wouldn't be surprising that beamformer is showing sources not apparent
> at sensor-level.
>
> On the other hand, beamformer results might show a bias towards the center
> of the brain unless normalized leadfields or pseudo-t/z are used (but just
> by looking at your plots this doesn't seem to be the case, sources seem to
> fall nicely over auditory cortices).
> Beamformer output can be also affected by highly correlated sources, but
> since you've used monoaural stimulation that shouldn't be a great deal here.
> It may be worth trying a different source estimation method and compare
> the results.
>
> Best wishes
> Marc
>
> On Sun, Dec 18, 2016 at 11:55 AM, Schoffelen, J.M. (Jan Mathijs) <
> jan.schoffelen at donders.ru.nl> wrote:
>
>>
>>
>> Begin forwarded message:
>>
>> *From: *Olga Sysoeva <olga.v.sysoeva at gmail.com>
>> *Subject: **ASSR and midline activity*
>> *Date: *17 December 2016 at 23:47:38 GMT+1
>> *To: *<jan.schoffelen at donders.ru.nl>
>>
>> Recently, I've got some unexpected results from localization of Auditory
>> Steady-State Response (ASSR) with Fieldtrip. The stimulation was monaural
>> 40 Hz clicks train (500 ms). In addition to the clear two clusters of
>> significant activity in left and right auditory cortex, I see big and
>> pronounced increase in ITPC (as well as total and evoked response) along
>> the midline structures (see Figure 1 as example of source reconstruction of
>> grandaverage of 40Hz ITPC values (N = 30). The monaural presentation of
>> 40Hz clicks into left ear). Can it be some artifact of the method I used
>> for localization? That was:
>>
>>        1. preprocessing/artefact detection;
>>
>>    1.
>>
>>    bandpass filtered form 35 to 45 Hz using Infinite Impulse Response
>>    (IIR) Butterworth two-pass filter of 4th order (forward and reverse),
>>    which results in a zero-phase shift of ERP signal;
>>    2.
>>
>>    Preprocessing of individual MRI/head model and so on construction;
>>    3.
>>
>>    Than the data were averaged for each ear separately, and covariance
>>    matrix was calculated based on the whole epoch;
>>    4.
>>
>>    *The output of this stage analysis was used for calculation the
>>    beamformer filter transformation from sensor to the source level. In
>>    particular I used Linearly Constrained Minimum Variance (LCMV).*
>>    5.
>>
>>    ITPC (as well as Evoked and total power) was calculate at each vxl.
>>
>>    a) we performed frequency decomposition using wavelet method based on
>>    Morlet wavelets for the frequency of 40 Hz for 200-500 ms post click onset.
>>    As the output we used complex Fourier spectra to be able to extract the
>>    ITPC information.
>>
>>    b) For each voxel the total power was calculated as mean of absolute
>>    value, and ITPC as sum of absolute value divided by the averaging, which
>>    takes into account the phase difference.
>>    6. Statistical analysis with permutation test : activity vs  baseline
>>    (midline cluster is also highly significant).
>>
>> I am also attaching my script for details.
>>
>> My doubts about the "reality" of this midline cluster came from
>> observation of the sensor level individual response (see picture from
>> Neuromag). By the way, here in Moscow MEG Center we use Elekta 306-channel
>> *“* Vectorview*”* , Neuromag System.
>>
>> Best Regards,
>>
>> Olga.
>>
>>
>>
>> _______________________________________________
>> fieldtrip mailing list
>> fieldtrip at donders.ru.nl
>> https://mailman.science.ru.nl/mailman/listinfo/fieldtrip
>>
>
>
>
> --
> Marc Recasens
>
> Tel.: +44 7845810006 <+44%207845%20810006>
> Tel.: +34 639241598 <+34%20639%2024%2015%2098>
>
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