Dear Li Bei,
Li Bei wrote:
>Dear Juan Jose Lull Noguera :
>
> Could you help me again?
>
> My experiment is designed like this:
> CACBCACBCACB ,where C means control,A means
>stimulus 1,B means stimulus 2, and where are 10 scans
>for each character
>
>(A,B,C),so there are 120 scans total .now i want to
>get the active region of stimuli A and B.
> In fmri design, i specify 3 conditions ,for A,B,C.
> Vector for A is [10 50 90]
> vector for B is [30 70 110]
> vector for C is [0 40 80]
> duration:10
> In "result",i defined T contrast to show active
>regions in condition 1(A),so i wrote weight vector for
>T contrast [1 0 -1],which means A - C,then i got these
>active regions. ---
>Question 1,could you please tell me whether procedures
>above are correct???
>
>
I think you meant C = [0 20 40 60 80 100] instead of [0 40 80]. If this
is true then the procedures described are ok.
>Question 2,what's the meaning of F-contrast with
>vecotor 1 0 0 ?
>
>
Hmm. I don't see a meaning to this contrast, because you don't compare A
to anything...
>Question 3,is there any way to get active regions of A
>with F-contrast?
>
>
Yes: you can apply an F [1 -1] contrast. This will give you activated
and also deactivated regions. If you want activated regions only, then
you should apply a t-test (F tests find differences in 'any' direction
while t-tests distinguish between directions).
>Question 4,what's the meaning of [1 2 3] in
>F-contrast?
>
>
This doesn't tell me anything. I think (I am not a statistician so this
could be incorrect, but this is what I remember) that it would find
voxels that have a greater estimated activation (signal level) in C
related to B and in B related to A.
>Question 5,in 'Result',there is always a predefined
>T-contrast,that is,"effects of interest".what's the
>vector of this T-contrast???
>
>
>
It isn't a t-contrast but an f-contrast. There are some predefined
contrasts in SPM when you estimate a model. If you hadn't defined your
control (or baseline) task then it would find areas activated for
stimulus A and also areas activated for stimulus B. As you have
explicitly defined your baseline condition, I think that it doesn't give
you important information.
> Those question puzzled me for a long time,thank
>you for your help!!
>
>
>
>
Cheers
Juan J
>Li Bei
>
>
>
>
> --- Juan Jose Lull Noguera <[log in to unmask]>
>的正文:
>
>
>>Hi Li Bei,
>>I recommend you to read a mail from Will Penny
>>written two days ago. The
>>messsage name is just "[SPM]".
>>If you have two conditions, c1 and c2 and you want
>>to see whether c1 is greater
>>than c2 then you should 'build' a [1 -1] t-contrast.
>>However, if you want to
>>see whether c2>c1 then you would build a [-1 1]
>>t-contrast (provided c1 is the
>>first condition in the design matrix, c2 the
>>second).
>>If you build an F contrast like [1 -1] it will test
>>whether there are
>>differences between c1 and c2, that is, c1>c2 and
>>also c2>c1. That's why you
>>see differences between a t and an F contrast.
>>Cheers
>>Juan J
>>
>>
>
>
>=====
>Bei Li
>Institute of Neuroinformatics
>Dalian University of Technology
>2 Ling Gong Rd, Dalian 116023, China
>Email:[log in to unmask]
>
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