[FieldTrip] how to set lambda for resting state lcmv-beamformer

[Ali Bahramisharif]

Hi Fred,

As far as I remember, lambda is used to regularize the covariance matrix.
I mean: regularized_covariance= XX'+lambda*Identity. To set it properly,
you need to look at the eigen-values of XX'. Normally you want to get rid
of the small eigen values. One way is to choose lambda to be 1/1000 of the
largest eigen-value.

Hope this helps.
Ali

>
> Dear fieldtrip users,
>
> I am trying to compute a spatial filter
> to localize alpha-band activity in an
> eyes closed resting state meg recording
> using the LCMV-method.
>
> The condition of my covariance matrix is > 1*10e6
> so I set lambda = 0.01. When I then plot the NAI
> I find two clusters of activity:
> one in the center of the head,
> one in the occipital cortex.
>
> In Exercise 5 of the tutorial, it is
> mentioned to set lambda to 1e-29 in order
> to suppresses the center of head artifact.
>
> I wanted to ask if anybody could tell me what the
> rationale was to set lambda = 1e-29?
> Is there a standardized way to compute lambda?
>
> Best regards,
>
> Fred
>
> --
> Frédéric Roux, PhD student
> Department of Neurophysiology
> Max Planck Institute for Brain Research
> D-60529 Frankfurt am Main
> Frederic.Roux at brain.mpg.de
> +49(0)69630183225
>
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