[FieldTrip] Coherent Source Region Suppression - beamformer

[נועה הכהן]

Thank you for the information! Indeed I do have zero lag correlation in my
auditory data. I will try to use your application.
Noa

‫בתאריך יום ה׳, 24 בספט׳ 2020 ב-12:42 מאת ‪Schoffelen, J.M. (Jan Mathijs)‬‏
<‪jan.schoffelen at donders.ru.nl‬‏>:‬

> Hi Noa,
>
> Adding to Sarang’s clarifying points: if you have a steady-state
> stimulation paradigm, zero-lag correlation of the bilateral auditory
> response is a problem. In that case you could use the method described in:
> https://doi.org/10.3389/fnins.2018.00711. The paper contains links to
> code (and data).
>
> Best wishes,
> Jan-Mathijs
>
>
>
>
>
> On 24 Sep 2020, at 10:27, Sarang S. Dalal <sarang at cfin.au.dk> wrote:
>
> Hi Noa,
>
> Thanks for your interest in our method. Just to clarify, our original
> approach was designed for time domain beamforming, but it seems you'd
> indeed need the one that Jan-Mathijs suggests for DICS / frequency-domain
> beamforming. But if you're still interested, our very original time domain
> implementation is still distributed with the Nutmeg package I used to be
> involved with:
>
> https://github.com/UCSFBiomagneticImagingLab/nutmeg/blob/master/beamformers/nut_Region_Suppression.m
>
> However, in our experience, the spatial correlation issue is less likely
> to occur for high frequency responses in general, since only a few
> milliseconds of delay between the left and right hemispheres is enough to
> substantially reduce the spatial correlation between them and allow the
> beamformer to successfully separate the sources.
>
> Furthermore, back in 2006, it was common to perform beamforming based on
> the sample covariance of the averaged evoked responses. As it turns out,
> this exacerbated the problem of bilateral correlated sources. Since then,
> most beamformer implementations (including FieldTrip, Nutmeg, and
> MNE-Python) compute the sample covariance using unaveraged trials by
> default; the trial-to-trial correlation between left and right auditory
> cortex is usually not nearly as high as it is in the average, which allows
> the sources to be successfully separated. Even with standard auditory
> evoked responses, I've found that the cancellation is much less likely to
> occur when the sample covariance is computed this way.
>
> Although I haven't tried DICS for auditory responses, we've had great
> results with localizing high gamma band auditory MEG responses using time
> domain beamforming together with the Hilbert transform for the frequencies
> of interest. High gamma band activity seems to be generally more difficult
> to resolve with EEG; that might be due to head model quality or simply
> lower sensitivity for the particular auditory cortex sources. To maximize
> EEG head model quality, I'd recommend using digitized electrode positions
> and BEM head models generated from segmented individual MRIs.
>
> Hope that helps, and I'd be happy to discuss further.
>
> Cheers
> Sarang
>
> On Thu, 2020-09-24 at 07:14 +0000, Schoffelen, J.M. (Jan Mathijs) wrote:
>
> Yes, this is implemented in FieldTrip, as per the following publication:
> https://doi.org/10.1016/j.neuroimage.2008.01.045
>
> The method to use is ‘pcc’ in ft_sourceanalysis. Alternatively, you can
> handcraft your dipole specific forward models, to include both the
> ‘leadfield’ for the dipole of interest + the ‘leadfield’ of the
> to-be-suppressed region. This requires some creative post-processing of the
> results, but that’s straightforward.
>
> Best wishes,
> Jan-Mathijs
>
>
>
>
> On 23 Sep 2020, at 11:26, ⁨נועה הכהן⁩ <⁨noa1hc at gmail.com⁩> wrote:
> Hello all,
> I am working on high dense eeg data, with auditory stimuli. I tried to use
> dics beamformer in order to localize the gamma waves, but due to the
> spatial correlation in the signal, I didn't get optimal results (as I
> understand it..). I read about method that deal with this issue- by using
> weights for the coherent sources (Modified Beamformers for Coherent
> Source Region Suppression, Sarang S. Dalal
> <https://www.ncbi.nlm.nih.gov/pubmed/?term=Dalal%20SS%5BAuthor%5D&cauthor=true&cauthor_uid=16830939> ).
> Are there any applications of this (or similar) method in fieldtrip?
> I will be glad to hear about this issue,
> thanks, Noa
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