[FieldTrip] Failure of LCMV beamformer for Neuromag VectorView planar gradiometers

russ port russgport at gmail.com
Fri Oct 7 21:40:56 CEST 2016


I realize that my previous email was far too long. In short:

I'm having some trouble localizing some auditory steady state elekta data using LCMV beamformer in fieldtrip. I'm localizing the magnetometers and gradiometers separately and while the magnetometers are giving good results the gradiometers are not (see attached ppt). I suspect that this is due to the gradiometer data matrix being rank difficient due to running maxFilter.

Does anyone have any suggestions on how to run LCMV beamforming on SSS’d elekta gradiometer data?


> On Oct 1, 2016, at 12:34 PM, russ port <russgport at gmail.com> wrote:
> Dear Fieldtrippers/Fieldtrippians
> I was hoping someone could help me with an issue I have come across during my analyses of a preliminary study. In brief, for the study a person was scanned on a CTF 275 channel biomagnetometer and then directly after on a 306 channel Elekta Neuromag VectorView platform. During the scans, the subject listened to a 40Hz amplitude modulated 500Hz tone (to generate a 40Hz Auditory Steady-state Response [ASSR]). In order to get the Elekta-derived data into a workable format, the Elekta-derived  was MaxFilter’d for SSS (not tsss). After this, to analyze the datasets in an analogous manner, all datasets (both CTF- and Elekta-derived datasets [planar gradiometers and magnetometers analyzed separately]) were artifact cleaned (artifact rejection for jump/muscle artifact, ICA rejection of EOG/ECG components - OF NOTE FOR ELEKTA DATA THE ICA OUTPUT WAS LIMITED TO THE RANK OF THE DATA [BECAUSE OF SSS NOTED ABOVE]). Then the data was averaged, and a LCMV beamformer (ipsi-lateral channels only) targeted at the subject’s right or left hemisphere Helsch’s Gyrus was used to generate a ASSR time course. Attached (within the powerpoint slide deck) are figures showing the results . For each figure, the left column shows data that is just read with with no artifact correction (incase the ICA routines were improperly deforming the data), and the right column show the results of the methodology described above. The first and second row is the time-locked data from all sensors, both broad band (1st row - and also what is passed to the LCMV beamformer), and band-passed for gamma-band activity (30-58Hz). The third and forth rows are similar, though show the virtual electrodes time course (orientated to the ASSR). Hopefully you would agree that the CTF and Elekta magnetometer virtual electrode time course/results seem very appropriate. On the other hand, the Elekta planar gradiometer time course seem “lack-luster” to say the best. I did try to look into this, and found that during the beamforming, both CTF and Elekta magnetometers are rank sufficient (rank= number of channels for that hemisphere). The planar gradiometers on the other hand are rank deficient, which based on my understanding of the steps of an LCMV beamformer may be quite problematic. Has anyone else experienced this before and/or have suggestions how to circumvent this issue of poor planar gradiometer results from a LCMV beamformer?
> <fieldtrip_LCMV_results.pptx>
> Best,
> Russ Port

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