I think this is the key, Olaf.
I apologise that my original email caused some confusion, because I was
not asking specifically about the temporal extension of SSS, but rather
the use of SSS generally.
I would also hope that the SSS components reflecting environmental noise
sources in the outer sphere are only a small part of the sensor space
spanned by the leadfield matrix, so their removal would have little
affect on that matrix.
However, my question remains: in order to compare "with and without"
leadfield correction (as Olaf suggests), how do I extract the necessary
correction (projection) from MaxFilter?
Advice from MaxFilter experts (ie Neuromag?) much appreciated....
Rik
> Hi again, and sorry: I was a bit too quick, there is of course no "null
> space" in sensor space of the leadfield. But the question how much the SSS
> components have in common with the leadfield still remains, and if the
> overlap is small, how big the effect of such a leadfield correction on
> source estimates really is. Has anyone tried with and without?
>
> Olaf
>
>
>
>> I would say one has to think about SSSt as a temporal filtering method -
>> and you don't correct your leadfield after low-pass filtering either, for
>> example. But this raises another interesting question: If SSS (with or
>> without ST) only removes activity from sources outside the sensor array,
>> it should only remove patterns that are in the null space of the leadfield
>> - i.e. no correction would be required. If it removes patterns that are
>> NOT in the null space of the leadfield, these sources could potentially be
>> generated inside the head (where the brain is) - i.e. it might remove
>> signal! I would hope that it's the former.
>>
>> Olaf
>>
>>
>>
>>> Burkhard -
>>>
>>> I thought so too, but another colleague thought this was not the case.
>>> So if the Neuromag experts don't give the definite answer, we could have
>>> a vote?
>>>
>>> ;-)
>>> Rik
>>>
>>> Burkhard Maess wrote:
>>>
>>>> Hi Rik,
>>>>
>>>> this is an interesting question - but I think the temporal projection
>>>> does not modify the spatially organized leadfield. SSSt takes out the
>>>> part of the data which correllates highly between both expansions, but
>>>> you can not describe it by a certain spatial pattern as in the case of
>>>> the SSP.
>>>>
>>>> Cheers,
>>>> Burkhard
>>>>
>>>>
>>>> Rik Henson wrote:
>>>>
>>>>> Dear Neuromeg -
>>>>>
>>>>> Could you let me know how I can correct my leadfields for prior SSSt?
>>>>> In other words, I have a leadfield matrix, L, of n sensors x p
>>>>> sources, and would like to extract some form of projection matrix
>>>>> from Maxfilter that I can apply to L in order to remove those
>>>>> components of the sensor data that have been removed by SSSt.
>>>>>
>>>>> Many thanks
>>>>> Rik
>>>>>
>>>>>
>>> --
>>>
>>> -------------------------------------------------------
>>> Dr Richard Henson
>>> MRC Cognition & Brain Sciences Unit
>>> 15 Chaucer Road
>>> Cambridge
>>> CB2 7EF, UK
>>>
>>> Office: +44 (0)1223 355 294 x522
>>> Mob: +44 (0)794 1377 345
>>> Fax: +44 (0)1223 359 062
>>>
>>> http://www.mrc-cbu.cam.ac.uk/people/rik.henson/personal
>>> -------------------------------------------------------
>>>
>>>
>> --
>>
>>
>
>
>
--
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DR RICHARD HENSON
MRC Cognition & Brain Sciences Unit
15 Chaucer Road
Cambridge, CB2 7EF
England
EMAIL: [log in to unmask]
URL: http://www.mrc-cbu.cam.ac.uk/people/rik.henson/personal
TEL +44 (0)1223 355 294 x522
FAX +44 (0)1223 359 062
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