Fwd: Qdec Questions - Freesurfer

30 Oct 2013


      Great - and this would be true for all models?
For example, in the attached graph, I'm looking at a region representing
increased cortical thickness for one group compared to the other (orange
blob, Q: "Does the average thickness differ between HC and SAD?" nuisance
factor: mean thickness). From this plot it appears that SAD > HC, but based
on your answer to the previous question I would think this region should
represent HC > SAD. Can you please explain this?
Thank you!
On Wed, Oct 30, 2013 at 7:33 AM, Douglas N Greve
greve@nmr.mgh.harvard.eduwrote:
...
In your case you have HC listed first, so red/yellow means HC>SAD, blue
means SAD>HC
doug
On 10/29/2013 11:03 PM, Ashley Shurick wrote:
...
Hi Doug,
Apologies, but I have a rather easy follow-up question that I can't
figure out:
How do you interpret the effect in each significant region? A previous
post suggests using ctrl + left mouse click on a blob and examining the
plot of the data. However, for the model I am looking at ("Does the
thickness-ERQ correlation differ between HC and SAD?") I still can't
determine if cortical thinning represents HC<SAD or SAD<HC. I'm attaching a
representative plot here.
Thank you,
Ashley
On Tue, Oct 29, 2013 at 10:25 AM, Ashley Shurick <
ashley.shurick@gmail.com <mailto:ashley.shurick@gmail.**comashley.shurick@gmail.com>>
wrote:
Thank you!


On Tue, Oct 29, 2013 at 10:19 AM, Douglas N Greve
<greve@nmr.mgh.harvard.edu <mailto:greve@nmr.mgh.harvard.**edu<greve@nmr.mgh.harvard.edu>>>

wrote:
    On 10/29/2013 01:10 PM, Ashley Shurick wrote:
    > Hi all,
    >
    > I'm running analyses in Qdec and want to verify a few things:
    >
    > 1. When comparing two groups (HC vs patients) I am including
    mean
    > cortical thickness as a covariate, using the following equation:
    >
    >
    > bh.thickness = (lh.thickness*lh.surfarea +
    rh.thickness*rh.surfarea) /
    > (lh.surfarea + rh.surfarea)
    >
    >
    > Is this the best way to calculate global mean thickness?
    Yes
    >
    >
    > 2. I want to regress out any effects of age, therefore I need to
    > demean the ages for my groups as a whole, and not perform this
    > calculation on each group separately, correct?
    Yes, in which case the test of the difference between groups
    will be
    done at an age equal to the mean of the ages.
    >
    > 3. I'm including questionnaires as a covariate of interest -
    do I need
    > to demean these values as well?
    When you do a test on a continuous covariate, that test will be
    unaffected by demeaning of the covariate. Demeaning will
    affect the
    difference between groups if you use a separate covariate for each
    group. If you have a single covariate across groups, demeaning
    will have
    no effect.

    doug
    >
    >
    > Thank you in advance for your help!
    >
    >
    > Ashley
    >
    >
    > --
    > Ashley A. Shurick
    > Ph.D. Candidate
    > Department of Psychology
    > Stanford University
    >
    >
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