Coherent sources

[Robert Oostenveld]

Hi,

On 3 Nov 2004, at 15:48, R. Sreenivasan wrote:

> Is it possible to compute coherence (sources)  for more than one
> reference
> dipoles (i.e, network) using Fieltrip?

It is not possible to compute it with the frequency implementation of
the beamformer (i.e. DICS). However, you could use the following
approach

- PREPROCESSING
- TIMELOCKANALYSIS using a bandpass filter (which is a hidden option)
in the frequency band of interest (not too narrow), with covariance=yes
and keeptrials=yes
- SOURCEANALYSIS with method=lcmv and singletrial=yes, with
cfg.grid.pos equal to the locations of interest
- FREQANALYSIS on the output of sourceanalysis, i.e. analyze the
virtual channel that was constructed on each location of interest with
output=powandcsd
- FREQDESCRIPTIVES

The sourceanalysis with method=lcmv and singletrial=yes will compute
the average covariance matrix from your data and use that to construct
the spatial filter. Subsequently the spatial filter is applied to each
trial seperately. The reason for averaging the trials prior to applying
the filters to the individual trials is that each individual trial
covariance is probably rank deficient, and will at least be pooirly
estimated resulting in poorly defined spatial filters. However, the
spatial filter can be determined robustly from the average.


best regards,
Robert

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Robert Oostenveld, PhD
Center for Sensory-Motor Interaction (SMI)
Aalborg University, Denmark

and

F.C. Donders Centre for Cognitive Neuroimaging
University Nijmegen
P.O. Box 9101
NL-6500 AH Nijmegen
The Netherlands

Tel: +31 (0)24 3619695
Fax: +31 (0)24 3610989
----------------------------------------------------------------------
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