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SPM  May 2011

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Subject:

Re: After estimation design matrix loses most regressors.

From:

Michael Thomas Smith <[log in to unmask]>

Reply-To:

Michael Thomas Smith <[log in to unmask]>

Date:

Tue, 17 May 2011 18:16:59 +0100

Content-Type:

text/plain

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text/plain (93 lines)

Thanks for the reply.
It seems likely it's something to do with my install of SPM, as Thomas  
was able to run exactly the same job on his computer and get the  
correct result. I'll reinstall SPM and try again. (by the way: I found  
adding extra regressors caused the left-out columns to change...all  
very weird).

Thanks again for the suggestions and help,

I'll be back with more questions if the reinstall doesn't work!

Mike.

Quoting Thomas Nichols <[log in to unmask]> on Tue, 17 May 2011  
12:51:23 +0100:

> Dear Mike,
>
> Before estimation, the displayed design matrix is original matrix X (with
> the exception that DCT drift basis elements aren't shown).  After
> estimation, the 'filtered and whitened' design matrix (xX.xKXs.X = K*W*X) is
> shown.  If certain columns appear to be zeroed out by whitening, that is
> because the corresponding scans were found to have super-high variance.
>  (See my earlier post on errors with PET data).
>
> I can't figure out why this is happening, though, as each session's data is
> scaled separately.  You can check out the scan-by-scan scale factors used to
> bring each session to a Grand Mean of 100 by looking at SPM.xGX.gSF; if
> there is crazy variation in this (e.g. super-low values for sessions 1 & 3)
> it would explain the effect.  Then the question is why are the computed
> globals so different over sessions... maybe crazy-large artifactual values
> in sessions 2, 4, 5,..,8?
>
> Hope this helps with the detective work!
>
> -Tom
>
>
> On Wed, May 11, 2011 at 12:57 PM, Michael Thomas Smith <
> [log in to unmask]> wrote:
>
>> I'm interested in the correlation between a time series and the voxels in
>> the brain. To find out more I made a GLM
>> (see http://www.sal.mvm.ed.ac.uk/images/problem1.png ).
>>
>> Before estimation it looks fine (I think?).
>>
>> During estimation however, 6 of the 8 columns are set to zero (see inset in
>> bottom-left corner of the linked image). Just sessions 1 and 3 output beta
>> images. There are no errors or warnings expressed during estimation, and the
>> matlab output appears ok.
>>
>> - I've confirmed the number of images in each session is equal to the
>> number of values in the .txt regressor files.
>> - I've tried adding conditions to each session (just a single event near
>> the start of each session) to see if it's the lack of conditions that's
>> upsetting SPM. (this didn't make any difference).
>> - I've rebuilt the design matrix from scratch by hand, using the interface,
>> but that didn't make any difference.
>>
>> Looking forward to any suggestions!
>>
>> Thanks,
>>
>> Mike Smith
>>
>> --
>> The University of Edinburgh is a charitable body, registered in
>> Scotland, with registration number SC005336.
>>
>
>
>
> --
> ____________________________________________
> Thomas Nichols, PhD
> Principal Research Fellow, Head of Neuroimaging Statistics
> Department of Statistics & Warwick Manufacturing Group
> University of Warwick
> Coventry  CV4 7AL
> United Kingdom
>
> Email: [log in to unmask]
> Phone, Stats: +44 24761 51086, WMG: +44 24761 50752
> Fax:  +44 24 7652 4532
>



-- 
The University of Edinburgh is a charitable body, registered in
Scotland, with registration number SC005336.

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