Beamformer time course

Marie Smith marie at PSY.GLA.AC.UK
Tue May 23 17:09:47 CEST 2006


I have tried the analysis as you suggest and although the analysis
appears to run I cannot find the single trial time courses in the
outcome of the second run of sourceanalysis.
In fact, it seems to output for each dipole location one value for
each of the power, the moment and the noise.

Should I enable the option singletrial = yes to determine the time
courses, and use the final two run method to obtain the correct power

Also, for information I am having some problems using the compiled
mex files with matlab 7 under unix.



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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