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.eduwrote:
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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