No no sorry for the misunderstanding. Either you correct by division with

subic_vol = average_eTIV_allsample * subic_vol / eTIV_subject

OR y0ou correct by regression. Only if you do regression, I’d use (eTIV_subject - average_eTIV_allsample) rather than eTIV_subject directly (and the same for all other variables: age, etc). Again: the result will be the same, but if you look at the residuals (plus the global intercept), demeaning will leave you with something that looks like the volume of a subiculum.

Cheers,

/Eugenio

Juan Eugenio Iglesias

Senior research fellow

CMIC (UCL), MGH (HMS) and CSAIL (MIT)

http://www.jeiglesias.com

From: <freesurfer-bounces@nmr.mgh.harvard.edu> on behalf of Steve Petersen <marfneuro@gmail.com>
Reply-To: Freesurfer support list <freesurfer@nmr.mgh.harvard.edu>
Date: Wednesday, April 22, 2020 at 12:39
To: "freesurfer@nmr.mgh.harvard.edu" <freesurfer@nmr.mgh.harvard.edu>
Subject: Re: [Freesurfer] Correlation between Hippocampal subfields - behavioural test

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Dear Eugenio,

Thank you for your quick response. Sorry but I am not completely sure if I understood correctly your response. What do you mean with "demeaning" the eTIV first?.

If I understood, a good choice would be to correct the subic_volume before perform the regression using the folliwng formula:

subic_vol = average_eTIV_allsample * subic_vol / eTIV_subject

Thank you.

Best,

Steve.

El mié., 22 abr. 2020 a las 17:35, Steve Petersen (<marfneuro@gmail.com>) escribió:

Dear Freesurfer experts,

I would like to perform a correlation between the volume of subiculum and some behavioural measures. Before making that correlation, is it necessary to correct the subiculum volume with the eTIV? I have thought of doing it by scaling the subiculum volume as follows:

subic_vol = mean_eTIV_group * subic_vol / eTIV_subject     Is correct??

mean_eTIV_group = eTIV average of the group to which the subject belongs.

Thanks in advance.

Best regards,

Steve.