Dear Giovanni,
[log in to unmask] wrote:
> Sir,
>
> I've got 2 questions about a very simple alertness task. I
>
> did a comparison of the functional anatomy between a
>
> intrinsic alertness "without warning stimulus" named T2 and a
>
> phasic alertness "presence of warning stimulus" named T3. I also
>
> used a sensorimotor control task named T1.
>
> I used SPM5, a blocked and categorical design and 3 conditions (T1, T2 and T3). The contrast T2
> vs T1 displays the intrinsic alertness whereas T3 vs T2 displays the phasic alertness. Whatever
> the contrast, many activated anatomical regions are quite similar.
>
> I measured in fMRI the reaction times (RTs) for each subject and for each condition. As expected,
> the reaction time is significantly shorter for T3 (effect of the warning stimulus).
>
> My questions are the following :
>
> 1) Can I enter the reaction times (in particular T2/T3 RTs) as covariables in the matrix design
> to see if the reaction times can change the activation pattern of each kind of alertness ?
>
> 2) How can I test a possible effect of the RT using SPM5 ?
>
> 3) Can we consider the RTs as a parametric modulator?
>
Yes. As you suggest, the best way of doing this is to use RT as a parametric modulator.
This will give you an extra regressor for each condition.
You can do this in SPM5 under Specify first level->fMRI model specification->Data & Design->
Subject/Session->Conditions->Condition->Parametric modulations. Simply enter a vector having as
many entries as the corresponding onsets vector ie. the RTs for that condition.
You can then test for RT effects separately for each condition (using appropriate
t or F contrasts), or collapsed over all conditions (using an F contrast).
Hope this helps.
Best,
Will.
>
> Thanks a lot for your time
>
> Best regards, Giovanni de Marco
>
>
>
--
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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