Dear spm-maillist and Nici
Do movement confounds then have to be centered around 0 or their mean??
Unless I am thinking wrong, the realigned image volumes are commonly
coregistered to the first image volume rather than the mean image volume.
The former creates confounds that do not center around 0 whereas the
latter do if I am thinking correctly. Are the former a problem for
entering as use specificied regression in the fMRI design step??
Even the famous faces / nonfamous faces example on
http://www.fil.ion.ucl.ac.uk/~wpenny/datasets/face-rep/SPM2/README-SPM2.txt
coregisters to the first image volume rather than the mean one.
Aren't hrfs often not mean centerted 0 as most of the activity is above
baseline?
Thanks
Jeff Lorberbaum
On Sat, 13 Dec 2003, Nici Wenderoth wrote:
> At 20:28 12.12.03 +0000, you wrote:
>
> Hi Kyle
>
> >Dear SPM list,
> >
> > I conducted an event-related study (TR=2.0 sec) in which
> >participants looked at pictures of common objects during three scanning
> >runs. I later had a separate group of participants rate the stimuli on
> >several dimensions (e.g., how easily a picture can be imagined). I now
> >want to see if the imagery ratings predict the intensity of activity within
> >the clusters of activity in the standard fixed effects analysis of the FMRI
> >data. I assume that this information should be entered as a user-specified
> >regressor in the design matrix. I have two questions though:
> >
> >First, should I mean correct the vector describing the imagery ratings?
> >The problem with mean correcting the scores is that some of the values are
> >now negative. In this case, I expect that activity should increase with
> >each picture presentation, but that it should be greater based on how
> >easily it can be imagined. When I mean correct, half of the scores are
> >negative, which might suggest deactivation for a given stimulus
> >presentation. While researchers often mean correct their regressors, is
> >mean correction always necessary?
>
>
> Yes, you should mean correct your regressors (as repeatedly suggested by
> the spm list). In this analysis you are looking at the correlation between
> your perception score and brain activity. That means for a positive
> correlation that brain activation should be low when the perception scores
> are low and high when the perception scores are high. Thus, for the
> correlation it does not matter whether scores are partly negative or not.
>
>
> >Second, if I do mean correct the scores, should I use the mean rating of
> >the stimuli within the run, or should I use the overall mean rating for the
> >stimuli? In other words, should I use the same mean value in all three
> >runs, or should I use a calculate a mean rating for just the stimuli in a
> >given run, and subtract that value from each individual picture's rating in
> >that particular run?
>
> mhhhhh, if you expect run specific effects (in other words, if you
> calculate an ANOVA on the perception scores, is there a significant effect
> of run?), I would enter the mean rating of the stimuli within the run. If
> not, I would average the values across runs to reduce the influence of
> outliers. however, i guess there could be more arguments in favour for the
> one or the other procedure.
>
> Regards, Nici
>
>
> >Thanks for any suggestions!
>
> Dr. Nici Wenderoth
> Motor Control Laboratory
> Dep. of Kinesiology
> K.U.Leuven
> Tervuurse Vest 101
> 3001 Heverlee
> Belgium
>
> E mail: [log in to unmask]
> Tel: 32 16 32 90 72
> Fax: 32 16 32 91 97
>
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