Dear Greve,

Thank you very much for your response. I am now a little more clear, although I still have some further questions.

According to your suggestion, I just tried to test whether there is a difference between group age. I included age as a continuous variable in qdec. After running, I got the results arising from the contrast 0 0 1 -1 (which is called does the thickness-age correlation differ between two groups?). That is, the results revealed the thinner in some brain areas. But when I checked the results relating to the comparision of groups (1 -1 0 0 ), the results were the same expect the opposite pattern. That is, the results became thicker in the same region.

Does this imply that there is an age effect? It means I could not use DOSS to do analysis, right?

Best,

Lickey

On Thu, Jan 9, 2014 at 12:32 AM, Douglas N Greve <greve@nmr.mgh.harvard.edu> wrote:

On 01/08/2014 04:28 PM, yaya ya wrote:
> Dear Freesurfer experts,
>
> I have a question about cortical thickness analysis using freesurfer.
> Since I don't know how to post my question in freesurfer mailing
> list, that is why I am writing to you to get some help. Hopefully, my
> email will not cause you any troubles.

Thanks for posting to the list.

>
> My question is : When I compared thickness from two groups (a patient
> group and a healthy contrl group), that is say I got the thinner in a
> subdivision of a particular brain region in patients than that in
> controls. But when I added age as a nusiance factors (not in the
> continuous covariate because I want to control for or regress out the
> age effect) in qdec, the results changed. In the same region, the
> thickness became thicker in the patients than that in the controls. I
> have no idea what happened. Which of them do I need to trust? The age
> between two groups has been matched.

It depends on several things. It might be that there is an interaction
between group and age. Try testing for a difference between the age
slopes of the two groups. If there is no difference, then re-run using
DOSS (DOSS was removed from QDEC so you'll have to create an FSGD file
and use mri_glmfit).

>
> In addition, when I do FDR correction on the results, the min in the
> threshold jumped to more than 5 , making all the clusters disappear. I
> think this correction is too high, unabling the genious effects to
> show up. How to deal with it?

Try using a cluster correction instead of FDR.
doug

>
> By the way, our lab's IT staffs install the lastest released
> freesurfer. But when I use the qdec, I could not find the option of
> DODS and DOSS, as discussed in the mailing list. I am wondering if it
> has been deleted in this new version.
>
> I really appreciate your help in advance.
>
> Best,
> Lickey
>
>

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Douglas N. Greve, Ph.D.
MGH-NMR Center
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