Different number of vertices for subjects with warped anatomy

Hi,

My goal is to compare the difference in power between two groups for all the vertices in the head model (using warped ICBM152 anatomies).

I have made a mixed head model consisting of the default cortex (constrained) and the thalamus (unconstrained). So in anatomy view, each of my subjects have a head model consisting of 16557 vertices (15002 for the cortex and 1555 for the thalamus). Subsequently I have warped each subject's anatomy using the polhemus head points. Then I created a source model (Minimum Norm) and computed the average power in a frequency band. This worked well for each individual, resulting in the figure below (this is one subject with 1 value for every vertex):
image

However, the number of values (when I click view file contents) is different from what I expected; 14475 in stead of 16557. Also, for every subject the number of values varies (14474, 14450, 14480, etc). Am I doing something wrong or do you know what causes the difference in number of values?

From the tutorial I understood that I should not have to project the results back on the default anatomy for the group analysis, because I'm using warped templates. However, because I had a different number of values for each subject, I did try this. This worked well, however the number of values again didn't make sense to me (15386) and I'm not sure how reliable this group average would be?

I hope my question is clear, if not please let me know!

Thanks,
Bart

Hi Bart,

These mixed head models are complicated to deal with, and impossible to use the context of a group analysis.

However, the number of values (when I click view file contents) is different from what I expected; 14475 in stead of 16557.

I guess 16557 is the number of the vertices on the surfaces (eg. the surface of the amygdala + the surface of the cortex), and 14475 is the number of source points (the vertices of the amygdala surface are not used as source points, but instead a regular grid of source points is defined inside the volume of the amygdala).

From the tutorial I understood that I should not have to project the results back on the default anatomy for the group analysis, because I'm using warped templates.

This hold only if you are using a surface head model, not a mixed head model.

I recommend you use simpler source models: either a surface head model or volume head model.
Instructions for group analysis with volume head models:
https://neuroimage.usc.edu/brainstorm/Tutorials/CoregisterSubjects#Volume_source_models

Hi Francois,

Thank you for your response! I will try the volume head model.

Thanks,
Bart