On 2/23/2021 11:37 PM, miracle ozzoude wrote:

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Thanks Doug. I appreciate it. I have more questions.

1) Aren't both models answering the same question regardless of which side of the equation cortical thickness or syndromes is placed?

Similar questions, but the results will not be identical. Eg, if cognition and thickness are identical numbers, then you can't solve Model 1 but you can solve Model 2. I was thinking you wanted to use the thickness from the entire cortex to estimate syndromes.

2) Also, I noticed in the mri_glmfit --help, it said that forward model 2 is inverted to solve for the regressor of interest. Correct if I'm wrong, does it mean if my matrix is 0 0 0 1 where 1 represents neuropsych syndrome, it will solve for it?

Yes, it will. But you will need to create an "image" of your neuropsych scores so that they are in each voxel.

The forward model is given by:

y = W*X*B + n

where X is the Ns-by-Nb design matrix, y is the Ns-by-Nv input data

set, B is the Nb-by-Nv regression parameters, and n is noise. Ns is

the number of inputs, Nb is the number of regressors, and Nv is the

number of voxels/vertices (all cols/rows/slices). y may be surface

or volume data and may or may not have been spatially smoothed. W

is a diagonal weighing matrix.

During the estimation stage, the forward model is inverted to

solve for B:

B = inv(X'W'*W*X)*X'W'y

3) Lastly, how do I extract the beta-values after running mri_glmfit-sim without matlab?

do you want a single beta as an overlay? Then use mri_convert and specify the frame you want with --frame (starts at 0). If you want values, you can run mri_convert beta.mgz --ascii beta.ascii

Many thanks,

Paul

On Tue, Feb 23, 2021 at 12:37 AM miracle ozzoude <miracooloz@gmail.com> wrote:

Hello Experts,

I've a question about mri_glmfit. I want to investigate the association between thickness and neuropsychiatric syndromes, controlling for age and cognition. Which model below is mri_glmfit performing?

Model 1
neuropsych syndromes ~ age + cognition + cortical thickness

or

Model 2

cortical thickness ~ age + cognition + neuropsych syndrome

Many thanks,

Paul
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