Beamformer time course

Jan Mathijs Schoffelen Jan.Schoffelen at FCDONDERS.RU.NL
Wed May 24 09:25:04 CEST 2006

Dear Marie,

Please have a look in the discussion's list archive. A question very similar
to yours has been answered: see the archive of May 2006, and follow the link
"source reconstruction". The discussion's list archive can be reached from
the website.



The dimensionality of the filter is a 3xN-matrix, with N the number of
sensors. The "3" corresponds with the x,y,z direction in space. In other
words: multiplying your data with the filter results in a 3xT (number of
time points) matrix, which represents a dipole's activity in 3-D space.

-----Original Message-----
From: FieldTrip discussion list [mailto:FIELDTRIP at NIC.SURFNET.NL] On Behalf
Of Marie Smith
Sent: Tuesday, May 23, 2006 7:47 PM
Subject: Re: [FIELDTRIP] Beamformer time course


Thanks again for all your help with my questions so far, it has been
very useful.

As an alternative method to determining the time course of
activation, I planned to use the filter as output from sourceanalysis
directly with the time locked data set.
However I am unsure what the three dimensions of the filter are.
Could you please clarify this for me.



On 23 May 2006, at 11:12, Robert Oostenveld wrote:

> Hi Marie,
> On 22 May 2006, at 17:18, Marie Smith wrote:
>> I have been trying out the various methods as discussed and have
>> come across a problem.
>> When I select cfg.singletrials = 'yes'  I get a number of error
>> messages saying
>> error('this option contains a bug, and is therefore not supported
>> at the moment');
>> I am using the toolbox  fieldtrip-20060516.
>> I noticed however, if I select cfg.rawtrials = 'yes' the function
>> will run.
> I recall disabling this functionality, since it was rarely used and
> since I was concerned that it would not be correct. I now had a
> more detailled look at the code.
> If you do rawtrial=yes, beamformer uses the single-trial covariance
> to construct the filter and also applies it to the single trial
> covariance/csd. That results in a very poorly estimated filter in
> each trial, hence projecting large amounts of noise and a lot of
> variance over trials. Therefore we found it not to be very usefull
> on real data, although conceptually and in the code it is correct.
> The idea behind singletrial=yes is that beamformer uses the average
> covariance to construct the filter and applies it to the single
> trial covariance/csd. The problem in the code however is that
> beamformer would use the averaged covariance/csd to estimate the
> power and not the single trial covariance/csd. The estimated single-
> trial timecourse would be correct, but the power would not be
> correct. Therefore I disabled that option in the code.
> The solution is something that you can do yourself in two
> subsequent runs of sourceanalysis: you can do sourceanalysis on the
> average (i.e. do not specify any of singletrial/rawtrial options)
> and specify keepfilter=yes. That will give you the filter, computed
> on the average covariance. Subsequently do cfg.grid=source2grid
> (source), and use the resulting grid (including the filters) in the
> second run of sourceanalysis, in which you specify
> cfg.rawtrial=yes. In the second run, the previous (based on average
> covariance) filters will be applied to the unaveraged data,
> resulting in the desired single-trial power and dipole moment.
> best regards,
> Robert

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