[FieldTrip] coherence normalization

Fri Dec 2 19:49:33 CET 2011

Hi Jan, Tom and the rest,

I agree with Tom, but would like to strongly emphasize that differences in power across conditions more often than not will affect the coherence landscape in a non-trivial way. This does not only count for power changes in the reference dipole, but also for changes in power for third party dipoles (i.e. any potential other source). Therefore the interpretation of the subtracted  coherence volumes should be done with care.

BW,

JM

On Dec 2, 2011, at 6:52 PM, Tom Holroyd (NIH/NIMH) [E] wrote:

> The solution I have adopted is to always look at coherence contrasts. Make two volumes using the same reference dipole in two different conditions, then subtract the volumes. The self-coherence of the reference will disappear. Mostly. Then use stats, like a U-test.
> 
> Jan.Hirschmann at med.uni-duesseldorf.de wrote:
>> Hi community,
>> Regarding this thread on suppressing the reference dipole from 2004, what is the current status? Has anybody found and implemented a recommendable way to project out activity from unwanted dipoles? Thank you for any comments/opinions!
>> Best,
>> Jan Hirschmann
>>   On 21 Oct 2004, at 17:23, Tom Holroyd wrote:
>> 
>>> / When running a coherence volume using a reference dipole, one/
>>> / naturally expects the coherence will be high around the reference/
>>> / dipole./
>>> / /
>>> / This effect tends to dominate the images./
>>> / /
>>> / Is there a way to normalize the coherence volume to eliminate/
>>> / this effect?  Perhaps by dividing by the coherence in a "control"/
>>> / state?/
>> Hi Tom,
>> The dominating effect of the refdip is indeed very problematic. I just
>> happened to have discussed this with Joachim Gross, and I have included
>> our email exchange below. Please first read that ...
>> Basically I agree with Joachim, and I don't trust the supdip that is
>> implemented in FieldTrip's sourceanalysis function. Better test and map
>> the significance of the difference in coherence between two conditions
>> using randomization of the trials before the coherence is beamed (that
>> is implemented in sourceanalysis + sourcestatistics).
>> Robert
>> --------------------------------------------------------------------
>> my question to Joachim was
>> ----------------------------------------------------------------------
>> Begin forwarded message:
>> 
>>> / From: Robert Oostenveld <//roberto at smi.auc.dk <http://mailman.science.ru.nl/mailman/listinfo/fieldtrip>//>/
>>> / Date: 1 October 2004 10:26:02 GMT+02:00/
>>> / To: Joachim Gross <//jgross at uni-duesseldorf.de <http://mailman.science.ru.nl/mailman/listinfo/fieldtrip>//>/
>>> / Subject: dipole suppression/
>>> / /
>>> / Hi Joachim,/
>>> / /
>>> / What I always still had to ask you is how you do supression of dipoles/
>>> / in DICS, especially in the case of coherence imaging. I have thought/
>>> / of two ways of projecting them out:/
>>> / /
>>> / 1) compute supdip leadfield and its projection on the COV/CSD matrix,/
>>> / then project it out of the COV/CSD matrix (which looses 2 or 3 from/
>>> / its rank)./
>>> / /
>>> / 2) compute supdip leadfield and add it to the leadfield of the dipole/
>>> / with which is scanned (scandip). Subsequently compute the source/
>>> / COV/CSD on those 6 leadfield components and select the 3x3 submatrix/
>>> / that corresponds with the scandip to continue the computations with./
>>> / /
>>> / Both methods don't really gave me very convincing results. A third/
>>> / approach would be to add the supdip leadfield to the (identity) noise/
>>> / matrix and project it through the filters. Then nai=pow/noise is/
>>> / corrected for the presence of the supdip, but that does not result in/
>>> / a supressed source coherence distribution. What is your idea or/
>>> / approach for this?/
>>> / /
>>> / best regards/
>>> / Robert/
>>> / /
>> ----------------------------------------------------------------------
>> and his answer (Joachim, I hope you don't mind me sharing this on the
>> list)
>> ----------------------------------------------------------------------
>> Begin forwarded message:
>> 
>>> / From: Joachim Gross <//jgross at uni-duesseldorf.de <http://mailman.science.ru.nl/mailman/listinfo/fieldtrip>//>/
>>> / Date: 14 October 2004 17:20:45 GMT+02:00/
>>> / To: "//robert.oostenveld at fcdonders.kun.nl <http://mailman.science.ru.nl/mailman/listinfo/fieldtrip>//"/
>>> / <//robert.oostenveld at fcdonders.kun.nl <http://mailman.science.ru.nl/mailman/listinfo/fieldtrip>//>/
>>> / Subject: dipole suppression/
>>> / /
>>> / Hi Robert,/
>>> / /
>>> / sorry for the delay./
>>> / /
>>> / The dipole suppression is indeed a complex issue./
>>> / We first implemented it because it facilitates visualization and the/
>>> / exact identification of the first/
>>> / strongest local maxima./
>>> / Nevertheless, it is quite dangerous because the map is (locally)/
>>> / distorted in a non-trivial way./
>>> / We are now trying to move away from suppressing the sources. I think/
>>> / it would be better to identify the/
>>> / significant local maxima (significance based on/
>>> / randomization/permutation)./
>>> / But what we are doing at the moment is your approach 3./
>>> / So we add the supdip leadfield to the noise covariance matrix and look/
>>> / at pow/noise./
>>> / /
>>> / For coherence we are basically doing the same thing./
>>> / So we divide the coherence map (or actually the map of cross spectral/
>>> / densities) by a noise map/
>>> / that peaks at the locations of the "unwanted" dipoles./
>>> / With this procedure we loose absolute coherence values./
>>> / This is not so important for us since we get the absolute values from/
>>> / the coherence and partial coherence spectra/
>>> / that are computed afterwards./
>>> / It works surprisingly well but should be used with care./
>>> / /
>>> / A better approach would be to map partial coherence (with the unwanted/
>>> / dipoles removed). But we have not implemented/
>>> / this so far./
>>> / /
>>> / Again, I think it is better to have regions of interest identified by/
>>> / their significance./
>>> / /
>>> / Joachim/
>>  ----------------------------------------------------------------------
>> 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
>> ----------------------------------------------------------------------
>> N.B. Starting from 1 September 2004, the University of Nijmegen has
>> changed its name to Radboud University Nijmegen. All web- and
>> email-addresses ending in ".kun.nl" should therefore be changed into
>> ".ru.nl". Please update your address book and links.
>>  Jan Hirschmann
>> MSc. Neuroscience
>> Insititute of Clinical Neuroscience and Medical Psychology
>> Heinrich Heine University Duesseldorf
>> Universitaetsstr.  1
>> 40225  Duesseldorf
>> Tel: 0049 - (0)211 - 81 - 18415
>> ------------------------------------------------------------------------
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>> fieldtrip at donders.ru.nl
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> 
> -- 
> "The test of a first-rate intelligence is the ability to hold two
> opposed ideas in the mind at the same time, and still retain the
> ability to function." — F. Scott Fitzgerald
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Jan-Mathijs Schoffelen, MD PhD 

Donders Institute for Brain, Cognition and Behaviour, 
Centre for Cognitive Neuroimaging,
Radboud University Nijmegen, The Netherlands

Max Planck Institute for Psycholinguistics,
Nijmegen, The Netherlands

J.Schoffelen at donders.ru.nl
Telephone: +31-24-3614793

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