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Subject:

Re: Longitudinal VBM analyses with CAT12.5

From:

"Flandin, Guillaume" <[log in to unmask]>

Reply-To:

Flandin, Guillaume

Date:

Mon, 14 Jan 2019 18:02:44 +0000

Content-Type:

text/plain

Parts/Attachments:

Parts/Attachments

text/plain (1 lines)

Dear Yann, 
 
1) This is expected given the inherent multicollinearity with paired 
t-tests. I would assume equal variance for this model. 
 
2) I'm not sure. The contrast you specified should be estimable. I could 
perhaps say more if you send me your SPM.mat. That said, for the main 
effect of group, you could instead compute the average effect within 
subject and enter these images in a two sample t-test. 
 
Best regards, 
Guillaume. 
 
 
On 11/01/2019 07:17, Yann Quidé wrote: 
> Hi all, 
>  
> Any thought on this? 
>  
> Thanks, 
>  
> Yann 
>  
>> On 9 Jan 2019, at 9:08 pm, Yann Quidé <[log in to unmask]> wrote: 
>> 
>> Dear all, 
>> 
>> I am analysing longitudinal VBM data (group1, n=10; group2, n=15; 2 time points) using CAT12.5 (r1363). 
>> 
>> 1) Looking within group for the effect of time my design matrix looks like attachment1… all boxes (but the one associated with TIV) below the matrix are grey, indicating that these data are not uniquely specified. As I am comparing the “same brains” over time, I believe this is ok; is that right? 
>> 
>> 2) I also wanted to look at the main effects of time, group, and their interaction using a 2x2 ANCOVA design (design in attachment2). I followed the CAT12 manual, and used  
>> ones(1,n)/n -ones(1,n)/n 0 ones(1,n1)/n1 -ones(1,n2)/n2 
>> to model the main effect of group (n=2; n1=15; n2=10). 
>> 
>> However, the creation of the contrast image crashes when using the batch mode, with the following error message: 
>> 
>> ======================================================================== 
>> Contrasts folder                        : ..tudinal_Time_by_Group 
>> 0.5         0.5        -0.5        -0.5           0    0.066667    0.066667    0.066667    0.066667    0.066667    0.066667    0.066667    0.066667    0.066667    0.066667    0.066667    0.066667    0.066667    0.066667    0.066667        -0.1        -0.1        -0.1        -0.1        -0.1        -0.1        -0.1        -0.1        -0.1        -0.1  <-  !invalid contrast 
>> Failed  'Contrast Manager' 
>> Error using spm_run_con (line 251) 
>> Error in contrast specification 
>> In file "/Users/y.quide/spm12/config/spm_run_con.m" (v7093), function "spm_run_con" at line 251. 
>> 
>> The following modules did not run: 
>> Failed: Contrast Manager 
>> ======================================================================== 
>> 
>> When creating the contrast image using contrast manager after selecting RESULTS, it works, BUT the result is surprising: the main effect of group is over the whole brain! 
>> 
>> Any idea what goes wrong? 
>> 
>> Thanks for your time. 
>> 
>> Cheers, 
>> 
>> Yann 
>> 
>> 
>> 
>> <Attachment1.png><Attachment2.png> 
>> 
 
--  
Guillaume Flandin, PhD 
Wellcome Centre for Human Neuroimaging 
UCL Queen Square Institute of Neurology 
London WC1N 3BG 

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