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

On 12/14/2021 5:07 PM, Laura Willers de Souza wrote:

External Email - Use Caution

Yes! Sorry for that.

And I'm resending my fsgd file!

Laura.

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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

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Answer by Douglas N. Greve:

Hello FreeSurfer Developers,

I want to perform an analysis of the difference between two groups
incortical thickness, having cortisol levels as covariate. This is
my model: cortical thickness ~ pathology (CU/CI) + CSFcortisol +
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?

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Answer by Douglas N. Greve: 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, 14 de dezembro de 2021 18:54 *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 Can you include the previous emails in your current email?

On 12/14/2021 3:46 PM, Laura Willers de Souza wrote:

...

External Email - Use Caution

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 https://secure-web.cisco.com/1qbucUNWhNS-2uW6Lox1w2bMK8EUHU0lm2XC7ywMcgAz1opgFdJ9TpOloRtF1uTmyVc70aldT5QkaELBu9kC0X6YiGpVUzZ3hV6ImEDrYAPF9VgAmDOudc6SysPVlhaUvzzGFirVYNLChsA_roK3faZNzYnWbdGPEJSgasor3qtQpKeAN7XvnQ4X24hWQfi35Rh8kF9yIsndHzOZGNxOdCH7U2CUMc9KxEL3N9u2-8weS-Fz01NHj2B_xMyBA8BlbS0n_wekioRXCPViGU3a_0zVQxE5Wu9sLPwFsQ3peECtbgAqHVmzWbyks9AhB3fWY/https%3A%2F%2Fsurfer.nmr.mgh.harvard.edu%2Ffswiki%2FFsgdf2G1V), 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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