Hi,all
Thanks for your helpful reply, Jason.
Then if I asume there is no rest condition during the experiment,
such as [high low high low...]. And I specify two columns in the model,
Then the contrast [1 0] reprsent 'high-low' or 'high-rest'? How to
decide the 'rest' here? What is the meaning of contrast [1 -1]?
Another question: I understanded that there are two ways to do
multi_subjects analysis. The first is to only do first level analysis;
The second is to two-levels analysis. Then what is the difference
between the contrast [1 -1] in the first way and the contrast [1 -1]
after finishing two-sample Test in the second. Thanks
Tianhao
>From: Jason Steffener <[log in to unmask]>
>To: Li Tianhao <[log in to unmask]>
>CC: [log in to unmask]
>Subject: Re: Some questions about design matrix in SPM99
>Date: Tue, 17 Apr 2001 21:17:10 -0400 (EDT)
>
>Hello,
> For part 1)
> You do not have to model the rest and the active condition. Only
>the active condition needs to be specified. If you model both active and
>rest in your design your results will be identical to the reults if you
>only specified the active condition. Reasons for the models producing
>identical results lie in the mathematics involved.
>
>Part 2)
> If you have a model consisting of this rest/active design for
>multiple subjects than each subject gets 2 columns. One is your on/off
>column and the other is the column of ones. A contrast looking at the
>active vs the rest condition for one subject is just 1 for that subject
>over their on/off column. You can create this contrast for each subject
>and then perform your conjunction analysis, if that is what interests you.
>
> good luck,
> Jason.
>
>On Tue, 17 Apr 2001, Li Tianhao wrote:
>
> > Dear SPMers,
> >
> > I am a new SPM user and I have some simple questions.
> > Any helps will be appreciated.
> >
> > (1) In SPM99, it is said that the 'rest' condition is implicit in
> > the design matrix. So we only build the design matrix and the
> > relative contrast as follows on stimulus [ high ,rest,high,rest...]
> > design matrix the number of condition:" 1"
> > [ 1 1
> > 0 1
> > 1 1
> > 0 1
> > .....] contrast [1 0]
> > Then what I understand is that the implicit 'rest' condition means that
> > SPM99 would add an implicit column in the deisgn matrix to estimate the
> > parameter of 'rest'. Because I do not know how to estimate the relative
> > parameter of 'rest' only by the above matrix. If my understanding is
>right?
> > or what is the truth?
> >
> > (2) If it is right, then I meet another question. Because the contrast
> > should be built as [1 -1] for different session/subjects in the
>conjunction
> > analysis according to many old email and reply. This is necessary for
>the
> > baselines are different among different sessions.
> > Then if I should build one special column for every 'rest' condition
> > in the design matrix as follows:
> > [ 1 0 1
> > 0 1 1
> > 1 0 1
> > ....] contrast[1 -1].
> > If so or not, why?
> >
> > Thanks again
> > Tianhao
> >
> >
> > _________________________________________________________________
> > Get your FREE download of MSN Explorer at http://explorer.msn.com
> >
>
>Jason R. Steffener, RTSV
>Department of Psychiatry, University Hospital, UMDNJ
>Office: (973) 972-1604
>Updated Webpage!!
>http://www.umdnj.edu/~steffejr
>
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