# LCMV questions

**URL:** <https://mne.discourse.group/t/lcmv-questions/2014>\
**Category:** Mailing List Archive (read-only)\
**Tags:** list-archive\
**Created:** [November 26, 2019, 4:34pm UTC](https://mne.discourse.group/t/lcmv-questions/2014 "2019-11-26T16:34:06Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![system](https://global.discourse-cdn.com/free1/uploads/mne/original/1X/85cc6bd2b69cb698a166dc6d880fb550510d0144.jpeg) [@system](https://mne.discourse.group/u/system)\
**Post date:** [November 26, 2019, 4:34pm UTC](https://mne.discourse.group/t/lcmv-questions/2014/1 "2019-11-26T16:34:06Z")

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I just started looking at the LCMV beamformer implementation in MNE and I'm  
a bit confused by the use of a 2 covariance matrices.

The implementations I'm familiar with (fieldtrip and ctf tools) use a  
single covariance. So to commute a differential beamformer you need to  
compute an average covariance (between the active and baseline time  
windows) to make the filter weights etc.

This is the approach taken in this tutorial.  
[http://www.fieldtriptoolbox.org/workshop/aarhus/beamformingerf/](http://www.fieldtriptoolbox.org/workshop/aarhus/beamformingerf/)

Does the MNE implementation do this already? Are the filter weights the  
same if I switch what I pass in for the data and noise covariances?  
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**Author:** ![system](https://global.discourse-cdn.com/free1/uploads/mne/original/1X/85cc6bd2b69cb698a166dc6d880fb550510d0144.jpeg) [@system](https://mne.discourse.group/u/system)\
**Post date:** [November 26, 2019, 4:39pm UTC](https://mne.discourse.group/t/lcmv-questions/2014/2 "2019-11-26T16:39:07Z")

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Hi Luke,

the data\_cov in the LCMV implementation serves the same purpose as  
Fieldtrip's data covariance matrix (i.e., it is used to compute the spatial  
filter). The noise covariance matrix is used to whiten the data, lead  
field, and covariance matrix. This is only mandatory if you supply data  
with several channel types (e.g., MEG and EEG or gradiometers and  
magnetometers) and is used to deal with scaling issues that arise with  
those different channel types.

Hope this helps,  
Britta

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**Author:** ![system](https://global.discourse-cdn.com/free1/uploads/mne/original/1X/85cc6bd2b69cb698a166dc6d880fb550510d0144.jpeg) [@system](https://mne.discourse.group/u/system)\
**Post date:** [November 26, 2019, 6:30pm UTC](https://mne.discourse.group/t/lcmv-questions/2014/3 "2019-11-26T18:30:17Z")

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Thanks Britta, that is what I was looking for.