Dear Douglas,

Again, thanks for the reply.

Please check if I understand correctly:

First I do the contrast [0 0 -1 1] to test if there is an interaction between diagnosis and cortisol; if there is, I do DOSS [1 -1 0], if there isnt’t, I continue with DODS.
Having no interaction, I do the contrast [1 -1 0 0] to check if only the diagnosis changes cortical thickness regardless of cortisol.

Did I interpret correctly?

And how do I check if cortisol changes cortical thickness regardless of diagnosis (assuming there is no interaction)?

Thanks again!

Best,

Laura Willers de Souza

Master Student in Biochemistry

Zimmer Lab - Federal University of Rio Grande do Sul

- Porto Alegre/RS - Brazil

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De: Douglas N. Greve <dgreve@mgh.harvard.edu>
Enviado: terça-feira, 14 de dezembro de 2021
Para: Laura Willers de Souza <laura.w.desouza@hotmail.com>
Cc: freesurfer@nmr.mgh.harvard.edu <freesurfer@nmr.mgh.harvard.edu>
Assunto: Re: [Freesurfer] Questions about contrasts

This contrast will do it: [1 -1 0 0]

But I would first test [0 0 -1 1] to see if the slopes are different. If not, then use [1 -1 0] with DOSS instead of DODS

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De: Laura Willers de Souza <laura.w.desouza@hotmail.com>
Enviado: terça-feira, 14 de dezembro de 2021 17:30
Para: Douglas N. Greve <dgreve@mgh.harvard.edu>
Cc: freesurfer@nmr.mgh.harvard.edu <freesurfer@nmr.mgh.harvard.edu>
Assunto: Re: Questions about contrasts

Dear Douglas,

Thank you so much for your answer!

Sorry, but I don't think I explained myself correctly in the previous email.

I want to analyze the difference in cortical thickness between two groups (CU and CI), adjusting for a covariate (cortisol CSF) and the interaction between them.

So my model would be: cortical thickness ~ diagnosis + CSF cortisol + diagnosis*CSF cortisol

Using the GLM DODS, the contrasts, according to the wiki example (MailScanner has detected a possible fraud attempt from "secure-web.cisco.com" claiming to be https://surfer.nmr.mgh.harvard.edu/fswiki/Fsgdf2G1V), would be:

To verify if only the diagnosis regression coefficient is significant (diagnosis changes cortical thickness regardless of cortisol):

1 -1 0 0

To verify if the regression coefficient of the interaction between diagnosis and cortisol is significant (the two change cortical thickness dependently):

0 0 1 -1

To verify if only the cortisol regression coefficient is significant (cortisol changes cortical thickness regardless of diagnosis):

0 0 0.5 0.5

Are these contrasts corret to represent the model we designed?

Thank you so much again!

Best regards,

Laura Willers de Souza

Master Student in Biochemistry

Zimmer Lab - Federal University of Rio Grande do Sul

- Porto Alegre/RS - Brazil

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De: Douglas N. Greve <dgreve@mgh.harvard.edu>
Enviado: terça-feira, 12 de dezembro de 2021 18:23
Para: Laura Willers de Souza <laura.w.desouza@hotmail.com>
Cc: freesurfer@nmr.mgh.harvard.edu <freesurfer@nmr.mgh.harvard.edu>
Assunto: Re: [Freesurfer] Questions about contrasts

The model is actually, two independent linear models:

thicknessCU = CUOffset + CUcortisol*CUslope

thicknessCI = CIOffset + CIcortisol*CIslope

There will be four regression parameters:

[CUOffset CUslope CIOffset CIslope]

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De: Douglas N. Greve <dgreve@mgh.harvard.edu>
Enviado: terça-feira, 12 de dezembro de 2021 10:48
Para: Laura Willers de Souza <laura.w.desouza@hotmail.com>
Cc: freesurfer@nmr.mgh.harvard.edu <freesurfer@nmr.mgh.harvard.edu>
Assunto: Re: [Freesurfer] Questions about contrasts

Hello FreeSurfer Developers,

I want to perform an analysis of the difference between two groups in cortical thickness, having cortisol levels as covariate.

This is my model: cortical thickness ~ pathology (CU/CI) + CSF cortisol + pathology*CSF cortisol

From the GLM DODS analysis I want to know if:

* the regression coefficient is significant only for the difference

between groups,

this could possibly mean two things: the difference in offset [1 -1 0 0] or the difference in slopes ([0 0 1 -1]

* or only for cortisol,

[0 0 0.5 0.5] will test whether the cortisol slope is different than 0

* or for both,

both what?

* or if the coefficient of the intercept between group and cortisol

is significant.

this does not make sense to me

Is my FSGD file (attached) correct? What would the contrasts look like?

---------------------------------------------------------------------------------------------------

De: Laura Willers de Souza <laura.w.desouza@hotmail.com>
Enviado: terça-feira, 09 de dezembro de 2021 19:50
Para: Douglas N. Greve <dgreve@mgh.harvard.edu>
Cc: freesurfer@nmr.mgh.harvard.edu <freesurfer@nmr.mgh.harvard.edu>
Assunto: Questions about contrasts

Hello FreeSurfer Developers,

I want to perform an analysis of the difference between two groups in cortical thickness, having cortisol levels as covariate.

This is my model: cortical thickness ~ pathology (CU/CI) + CSF cortisol + pathology*CSF cortisol

From the GLM DODS analysis I want to know if:

the regression coefficient is significant only for the difference between groups,
or only for cortisol,
or for both,
or if the coefficient of the intercept between group and cortisol is significant.

Is my FSGD file (attached) correct? What would the contrasts look like?

Thanks in advance!

Laura Willers de Souza

Master Student in Biochemistry

Zimmer Lab - Federal University of Rio Grande do Sul

- Porto Alegre/RS - Brazil