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Hi Freesurfer group:
I have been using ICV as a covariate in all my analyses. Can CORTEX VOLUME be a better measure of covariance? If not, why?
Thanks,
Alan
This is from chatgpt, which actually gave a pretty good answer this time
Why ICV is used as a covariate
*
*ICV reflects head/brain size*: It is established early in life and remains relatively stable in adulthood.
*
*Controls for global scaling effects*: If you’re comparing cortical or subcortical structures across individuals, larger heads naturally tend to have larger absolute volumes. ICV adjustment removes this confound so differences reflect local effects, not just overall size.
*
*Standard practice*: ICV is widely used in structural MRI studies for this purpose because it is a stable “baseline” measure.
------------------------------------------------------------------------
Why cortex volume is /not/ a good substitute
*
*Not independent*: Cortex volume itself is strongly influenced by disease, development, and aging. Using it as a covariate risks removing real biological signal related to the outcome you care about.
*
*Collinearity problems*: If your dependent variable is cortical thickness, surface area, or subcortical volume, adjusting for cortex volume introduces statistical dependencies (since they’re not independent measures). This can distort your estimates.
*
*Less stable than ICV*: Cortex volume can shrink over time due to atrophy or expand during development, making it unsuitable as a baseline scaling factor.
------------------------------------------------------------------------
When /might/ cortex volume be useful?
*
If your *research question is specifically about relative differences among cortical subregions* (e.g., “is region X disproportionately reduced relative to total cortex?”). In that case, cortex volume could serve as a /denominator/ or scaling reference, but not a general-purpose covariate.
*
Even then, most researchers would report both /absolute/ and /relative/ measures, with ICV adjustment kept separate.
On 9/18/2025 5:55 PM, Alan Francis wrote:
External Email - Use Caution
Hi Freesurfer group:
I have been using ICV as a covariate in all my analyses. Can CORTEX VOLUME be a better measure of covariance? If not, why?
Thanks,
Alan
-- /|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|/ /* */Dr. Alan N. Francis Assistant Professor of Neuroscience Director: Neuroimaging & Neuromodulation Research Lab Institute of Neuroscience University of Texas, School of Medicine Rio Grande Valley, Texas.
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Thank you, Doug.
On Sun, Sep 21, 2025 at 4:06 PM Douglas N. Greve dgreve@mgh.harvard.edu wrote:
This is from chatgpt, which actually gave a pretty good answer this time Why ICV is used as a covariate
*ICV reflects head/brain size*: It is established early in life and remains relatively stable in adulthood.
*Controls for global scaling effects*: If you’re comparing cortical or subcortical structures across individuals, larger heads naturally tend to have larger absolute volumes. ICV adjustment removes this confound so differences reflect local effects, not just overall size.
*Standard practice*: ICV is widely used in structural MRI studies for this purpose because it is a stable “baseline” measure.
Why cortex volume is *not* a good substitute
*Not independent*: Cortex volume itself is strongly influenced by disease, development, and aging. Using it as a covariate risks removing real biological signal related to the outcome you care about.
*Collinearity problems*: If your dependent variable is cortical thickness, surface area, or subcortical volume, adjusting for cortex volume introduces statistical dependencies (since they’re not independent measures). This can distort your estimates.
*Less stable than ICV*: Cortex volume can shrink over time due to atrophy or expand during development, making it unsuitable as a baseline scaling factor.
When *might* cortex volume be useful?
If your *research question is specifically about relative differences among cortical subregions* (e.g., “is region X disproportionately reduced relative to total cortex?”). In that case, cortex volume could serve as a *denominator* or scaling reference, but not a general-purpose covariate.
Even then, most researchers would report both *absolute* and *relative* measures, with ICV adjustment kept separate.
On 9/18/2025 5:55 PM, Alan Francis wrote:
External Email - Use CautionHi Freesurfer group:
I have been using ICV as a covariate in all my analyses. Can CORTEX VOLUME be a better measure of covariance? If not, why?
Thanks,
Alan
-- *|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|~|*
Dr. Alan N. Francis Assistant Professor of Neuroscience Director: Neuroimaging & Neuromodulation Research Lab Institute of Neuroscience University of Texas, School of Medicine Rio Grande Valley, Texas.
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