My apologies for such a basic question, but does given my design:

>> Inputs       Group  EV1  EV2
>> controlsubj1  1     1     .545
>> controlsubj2  1     1     .463
>> controlsubj3  1     1     .445
>> patientsubj1  1     1     .335
>> patientsubj2  1     1     .377
>> patientsubj3  1     1     .399
>>
>>                        EV1 EV2
>> allsubj, grp mean     [1    0]
>> poscorrel with FA    [0    1]
>> negcorrel with FA    [0   -1]

do I demean EV2 by subtracting 0.427 (ave of EV2) from each individual 
subject's EV2 val? Or do I simply change my contrasts to the following?

                         EV1 EV2
allsubj, grp mean     [1    0]
poscorrel with FA    [-1   1]
negcorrel with FA    [-1   -1]

Many thanks for the continued help,
Lara

On Tue, 9 Dec 2008 05:15:57 +0000, Steve Smith <[log in to unmask]> 
wrote:

>Nearly, but you should subtract the mean of EV2 so that it has zero
>mean.
>Steve.
>
>
>
>On 8 Dec 2008, at 22:05, Lara Foland wrote:
>
>> Thanks for your feedback.
>>
>> So to be clear, one way to look at the correlation of FA with fMRI
>> is to put
>> everyone in group 1. Can you confirm that the following setup is
>> appropriate?
>>
>>
>> Inputs       Group  EV1  EV2
>> controlsubj1  1     1     .545
>> controlsubj2  1     1     .463
>> controlsubj3  1     1     .445
>> patientsubj1  1     1     .335
>> patientsubj2  1     1     .377
>> patientsubj3  1     1     .399
>>
>>                        EV1 EV2
>> allsubj, grp mean     [1    0]
>> poscorrel with FA    [0    1]
>> negcorrel with FA    [0   -1]
>>
>> Again, thanks so much,
>> Lara
>>
>> On Fri, 5 Dec 2008 07:39:29 +0000, Steve Smith <[log in to unmask]>
>> wrote:
>>
>>> Hi - a couple of problems with this:
>>>
>>> - If you specify different group memberships for higher level FEAT
>>> analysis, the different groups need to have "separable" designs, so
>>> you would either need to put eveyone in group 1 (which doesn't
>>> invalidate the model, just possibly makes estimation a little less
>>> efficient), or split EV3 into two EVs, one for each group. In any
>>> case:
>>>
>>> - COn3 and 4 are not correct - a basic test of correlation with FA,
>>> using your current model, averaged across two groups, would be [0 0
>>> 1]. However, for reasons above, AND in order to allow you to test for
>>> _differences_ in the FA correlation between the groups, you should
>>> split EV3 into two, one for each group, and then test for within
>>> group
>>> correlations with [0 0 1 0], [0 0 0 1], etc., and for differences
>>> between the groups with [0 0 1 -1], etc.
>>>
>>> - You should subtract the mean of the FA values before putting in the
>>> model. In the case of splitting into two EVs, you should demean each
>>> group's FA values separately, and for each group's FA EV, the other
>>> group's entries will be 0s.
>>>
>>> Cheers.
>>>
>>>
>>> On 4 Dec 2008, at 23:51, Lara Foland wrote:
>>>
>>>> Dear FSLers,
>>>>
>>>> I would like some clarification regarding contrast setup for an
>>>> unpaired t-test
>>>> with additional EV for correlation.
>>>>
>>>> I am conducting a t-test on fmri images for subjects in two groups:
>>>> patients
>>>> and healthy. I am interested in the correlation of any fMRI
>>>> differences with
>>>> differences in FA values (taken from an ROI within a DTI scan for
>>>> each
>>>> subject).
>>>>
>>>> My design matrix is set up like the two sample t-test example in the
>>>> FEAT
>>>> manual page but with an additional EV containing the FA value for
>>>> each
>>>> subject.
>>>>
>>>> Inputs       Group EV1 EV2 EV3
>>>> controlsubj1  1     1    0   .545
>>>> controlsubj2  1     1    0   .463
>>>> controlsubj3  1     1    0   .445
>>>> patientsubj1  2     0    1   .335
>>>> patientsubj2  2     0    1   .377
>>>> patientsubj3  2     0    1   .399
>>>>
>>>>
>>>>                      EV1 EV2 EV3
>>>> Con1 con>pts      [ 1   -1   0 ]
>>>> Con2 pts>con      [-1    1   0 ]
>>>> Con3 con>ptsmod [ 1   -1   1 ]
>>>> Con4 pts>conmod [-1   1   -1 ]
>>>>
>>>>
>>>> Con1 and Con2 are the contrasts to test for fMRI difference between
>>>> groups. Con3 and Con4 are intended to test for fMRI change between
>>>> groups
>>>> that also correlate with a difference in FA. Are Con3 and Con4
>>>> set up correctly?
>>>>
>>>> Many thanks in advance,
>>>>
>>>> Lara
>>>>
>>>
>>>
>>> -----------------------------------------------------------------------
---
>> -
>>> Stephen M. Smith, Professor of Biomedical Engineering
>>> Associate Director,  Oxford University FMRIB Centre
>>>
>>> FMRIB, JR Hospital, Headington, Oxford  OX3 9DU, UK
>>> +44 (0) 1865 222726  (fax 222717)
>>> [log in to unmask]    http://www.fmrib.ox.ac.uk/~steve
>>> -----------------------------------------------------------------------
---
>> -
>>
>
>
>--------------------------------------------------------------------------
-
>Stephen M. Smith, Professor of Biomedical Engineering
>Associate Director,  Oxford University FMRIB Centre
>
>FMRIB, JR Hospital, Headington, Oxford  OX3 9DU, UK
>+44 (0) 1865 222726  (fax 222717)
>[log in to unmask]    http://www.fmrib.ox.ac.uk/~steve
>--------------------------------------------------------------------------
-