Fwd: [FIELDTRIP] problems with DICS

[Nathan Weisz]

ok ... we found the problem why the leadfields differ. apparently the
older default for cfg.reducerank = 2. newer fieldtrip versions have
cfg.reducerank = 3.

n


On 19.11.2008, at 14:30, jan-mathijs schoffelen wrote:

>
>
> Begin forwarded message:
>
>> From: jan-mathijs schoffelen <j.schoffelen at psy.gla.ac.uk>
>> Date: November 19, 2008 1:13:22 PM BST
>> To: nathanweisz at me.com
>> Subject: Re: [FIELDTRIP] problems with DICS
>>
>> Dear Nathan,
>>
>> Are you using precomputed leadfields? The reason I ask is because
>> there could be a discrepancy between the assumed order of the coils
>> in your leadfields, and the coil-order in your data. The issue with
>> the BTI system is that the channel order is somewhat erratic
>> (references ending up all over the place, and no nice alphabetical
>> ordering of the magnetometer coils). Prepare_leadfield (the low-
>> level function which computes the leadfield) just computes the
>> solution to the forward model for the list of sensors in the input,
>> the order of which is specified by the order in the data, or by the
>> order in the gradiometer structure, if no data is supplied.
>> I recently (about a month ago) made a change in bti2grad, which
>> changed the order of the coils in the grad-structure. Initially, I
>> thought it would be nice to have them ordered alphabetically, but
>> this led to problems later on when using precomputed leadfields
>> (and making implicit assumptions about the matching sensor order in
>> both data and gradiometers). It could be that your problems are
>> related to this.
>> On the other hand: could this be replicated in other datasets? How
>> many trials is your csd-matrix based on? Isn't there any hint of a
>> bilateral temporal decrease in alpha activity?
>>
>> Yours
>>
>> Jan-Mathijs
>>
>>
>>
>> On Nov 19, 2008, at 11:00 AM, Nathan Weisz wrote:
>>
>>> hi,
>>>
>>> i'm not sure whether the following question is a fieldtrip-related
>>> question or a rather general question.
>>>
>>> we use a 148 sensor BTI system.
>>> since a couple of days we're struggling with a data-set in which
>>> we'd like to localize auditory cortical alpha desynchronizations.
>>> on a sensor level they are clearly observable (see attached
>>> Pitcure 2). however when running DICS it seems like the brain is
>>> more or less increasing power post-stimulus.
>>> what makes me wonder is that it's exactly the same script (i could
>>> provide more details of course) which worked very successfully
>>> previously. our notion was initially that something is weird with
>>> the leadfield. but -illegally- testing it with a leadfield from
>>> another subject where things worked out basically gave more or
>>> less the same picture. now we assume that something is fishy with
>>> the data itself, that is not clearly observable when looking at
>>> the data on a trial-by-trial basis, leading to bad spatial
>>> filters. any suggestions how this could be diagnosed?
>>>
>>> are there any other suggestions? or has anybody had similar
>>> problems lately?
>>>
>>> cheers,
>>> n
>>>
>>>
>>>
>>>
>>>
>>> <Picture 2.png>
>>>
>>> <Picture 1.png>
>>>
>>>
>>> --------------------------------------------
>>> Dr. Nathan Weisz
>>>
>>> OBOB-Lab
>>> University of Konstanz
>>> Department of Psychology
>>> P.O. Box D23
>>> 78457 Konstanz
>>> Germany
>>>
>>> Tel: ++49 - (0)7531 - 88 45 84
>>> Email: nathan.weisz at uni-konstanz.de
>>> Homepage: http://www.uni-konstanz.de/obob
>>>
>>> "Nothing shocks me. I'm a scientist." (Indiana Jones)
>>>
>>> ----------------------------------
>>>
>>> The aim of this list is to facilitate the discussion between users
>>> of the FieldTrip toolbox, to share experiences and to discuss new
>>> ideas for MEG and EEG analysis.
>>>
>>> http://listserv.surfnet.nl/archives/fieldtrip.html
>>>
>>> http://www.ru.nl/fcdonders/fieldtrip/
>>>
>>
>
> ----------------------------------
>
> The aim of this list is to facilitate the discussion between users
> of the FieldTrip toolbox, to share experiences and to discuss new
> ideas for MEG and EEG analysis.
>
> http://listserv.surfnet.nl/archives/fieldtrip.html
>
> http://www.ru.nl/fcdonders/fieldtrip/
>


----------------------------------
The aim of this list is to facilitate the discussion between users of the FieldTrip  toolbox, to share experiences and to discuss new ideas for MEG and EEG analysis. See also http://listserv.surfnet.nl/archives/fieldtrip.html and http://www.ru.nl/fcdonders/fieldtrip.
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