[FieldTrip] Fwd: ASSR and midline activity

Marc Recasens recasensmarc at gmail.com
Mon Dec 19 13:29:35 CET 2016

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

Best wishes

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
Tel.: +34 639241598
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