Dear Li Bei,
RE DCM analysis:
When looking at the results of an SPM analysis
selecting different contrasts allows you to look at
maps of different effects.
In the data on the web, it was an arbitrary choice to
look at the effect of motion. You could use other contrasts
to look at other effects.
Looking at 'results', however, does not cause any VOI
files to be written. The use of contrasts in extracting
VOIs is a separate issue.
Here, it is useful to specify a contrast so that
effects of no interest can be removed. Think of this
as a filtering. So, if I remember correctly, an
F-contrast over the effects of Photic, Motion
and attention was used as we are interested in
modelling *all* of these effects with DCM.
If we were only interested in modelling a subset of these
effects we would have used an F-contrast spanning only
that subset.
RE GLM analysis:
Perhaps you could simplify your design by having only one
'control' variable (for A and B), instead of 3 (control for A, control for B,
control for A and B). I guess its possible that [1 -1 0 0 0 0]
could also be formed by a combination of the other regressors (which
would result in an invalid contrast) - but
I have'nt worked this out in detail.
Best wishes,
Will.
Li Bei wrote:
> Dear Penny:
> Thank you for you help,you are so kind!
> Sorry for bringing you so much troubles.
> I have another 2 quesion :)
> QUESTION 1
> BEGIN
> What's the difference of VOI in chosing different
> F-contrast in DCM instruction published on you
> websit,the Result section.
> In you example,you chosed Motion Condition ,and
> defined VOIs,but i found,value series of VOI are
> different in different
> F-contrast(Photic,Motion,Attention),why you chose
> Motion in you example?? What will happen if you chose
> Photic and define VOIs?
> END
>
>
>
> QUESTION 2
> BEGIN
> This Question is about fMRI modelling :)
>
> My exp. is designed like this
> CACBCACBCACB (12 total) and A for stimuli A,B for
> stimuli B,C for control
> there is 10 scans for each character ,so there are 120
> scans totally
>
> in fMRI Design Stage,i set the paramaters like this#:
> scans per session[120]
> Number of conditions[6]
>
> name for condition 1 [StimuliA]
> Vector [10 50 90]
>
> name for condition 2 [StimuliAControl] //Control
> Before Stimuli A
> vector [0 40 80]
>
> name for condition 3 [StimuliB]
> vector [30 70 110]
>
> name for condition 4 [StimuliBControl] //Control
> Before Stimuli B
> vector [20 60 100]
>
> name for condition 5 [StimuliA+B] //Stimuli A and
> B
> vector [10 30 50 70 90 110]
>
> name for condition 6 [StimuliA+BControl] //all
> controls before stimuli
> vector [0 20 40 60 80 100]
>
> then in fMRI Data stage,i chose those 120 picture ,
> and after Estimate#,i clicked Result
>
> then i chose t constract
> I want StimuliA - StimuliAControl ,and get the
> activation of StimuliA
> So i wrote like this: constract vector : 1 -1 0 0 0
> 0#,but the system told me that is an invalid vector!
> Why? i'm so warry about this.
> END
>
> Now i'm an undergraduate student, and i will apply a
> foreign university in countries out of China this
> september , I have to publish at least 2 papers before
> my application in order to get a good university,Then
> i have a relatively bigger change to have an
> archivment like you've got, time is not enough for me,
> so ,thank you for you help !!!
> Thank you!!
>
>
> My MSN:[log in to unmask],you can contact me if you
> want to give me more help,haha ;)
>
>
> Best Wishes
>
> BeiLi
>
>
> --- Will Penny <[log in to unmask]> 5DU}ND#:
>
>>Dear Li Bei,
>>
>>Ideally, for a DCM analysis, you should have at
>>least two factors.
>>
>>One would be a 'driving' factor - factor A,
>>typically
>>a series of events - though these could be blocked.
>>
>>The other would be a 'modulatory' factor - factor B,
>>typically
>>blocked.
>>
>>DCM is then primarily used to attribute changes in
>>effective connectivity to factor B.
>>
>>You should have enough variables in your
>>design to set up a DCM.
>>
>>You could specify D = ---AAABBB---AAABBB, ie
>>A or B. This would be a driving input.
>>
>>Your modulatory input would then be eg. B.
>>
>>Best,
>>
>>Will.
>>
>>
>
>
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>
--
William D. Penny
Wellcome Department of Imaging Neuroscience
University College London
12 Queen Square
London WC1N 3BG
Tel: 020 7833 7475
FAX: 020 7813 1420
Email: [log in to unmask]
URL: http://www.fil.ion.ucl.ac.uk/~wpenny/
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