Tutorial

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In this section of the tutorial we will apply Linearly Constrained Minimum Variance Beamformer reconstruction. See [1] for more information on beamformer approach.

• Go to the BST Main window ->Inverse Solutions -> Interactive to enable the Data Viewer.
• To limit the reconstruction in the 20-60ms time range (where the stimulus response is apparent in the time courses), choose Start: 20 and Stop: 60. The Step should read 0.80
• In the Analysis section select Cortical LCMV Beamformer in the drop down menu. This will open the LCMV Beamformer window.
• Realize that the text boxes for Start and Stop values for the baseline are inactive since Filter Output is selected as output format. Hint: You should fill out these boxes with appropriate values for baseline region if you select Normalized Output or Neural Index as output format. Filter Output is the output of the narrowband spatial filter, Neural Index is the Source Power normalized by the noise power, where source power and noise power are the estimated spatial spectrum of the data and the baseline respectively. Normalized output is the filter output that is normalized by the standard deviation of the baseline region.
• Default value for Tikhonov regularization parameter is 10, which selects a lambda value that is 10% of the maximum singular value of the data and noise covariances as needed.
• Select the neural index as output format and select the baseline region between -49.60 and 20 ms.

• Press the Go button to run the LCMV beamformer algorithm for these parameters.
• Surface Manager should open and and Envelope Viz should display the cortical surface overlayed with the current density map. Increase the threshold to observe the activation on the left hemisphere very close to the location observed by Minimum Norm Imaging Reconstruction. Hint: Neural Index output creates single detection map and doesn't change over time. Therefore changing the position of the slider will not have an effect on the cortical map. To observe the time series, select the output format Filter Output or Normalized Output.

The results are stored in the C:\BSTDemoData\Study\somDemo_av.ds\BrainStorm\ folder in the files somDemo_av_results_Trial1_LCMVBF_MEG_(time created).mat.

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