krista kelly wrote:
Hi, just wondering if anyone has any advice on the following questions:
I'm using QDEC to analyze cortical thickness differences between groups and I have a few questions:
- There are a few variables that I need to control for. First
variable is scanner - I used two different scanners of different field strengths (1.5T GE and 3T Siemens) so I wanted to control for scanner. I also wanted to control for age and gender. Here's my ideal design setup:
discrete variables - groups, scanner, gender nuisance variable - age
contrast would be 1 -1 0 0 0 0
I realize that scanner, gender, and group are all discrete variables, and that age would be a continuous one. However, in the design tab, I am not able to select all 3 discrete variables (groups, scanner, and gender). Should I designate scanner or gender as a nuisance variable instead? Would it matter which one I use as the nuisance variable? Possible setup:
discrete vars - groups, scanner nuisance vars - age, gender
contrast - 1 -1 0 0 (Correct?)
Unfortunately, you cannot use QDEC if you have three groups. You'll have to create an FSGD file and contrasts by hand, then run mri_glmfit. Do a search for FSGD on our wiki and go to the examples page. I think there is an example there for what you want to do.
- If I use DODS, should I demean the ages if age is a nuisance variable?
You should actually test whether there is an interaction between age and your group contrast of interest (using the DODS model). If there is not interaction, then switch to a DOSS where demeaning does not matter.
- If I use a smoothing kernel during the analysis, then create an ROI
and map that ROI to each individual subject, will the resulting .stats file in each subjects stats folder reflect the smoothed data or does mapping onto each ROI just extract the raw unsmoothed data from each vertex?
It depends on how you do the extraction. If you are using mris_anatomical_stats on each individual's unsmoothed data, then it will reflect the unsmoothed.
doug
Thanks! Krista
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