Dear experts,

I would like to show the across-subject overlap of voxels by creating a map with color coding indicating the number of subjects activating corresponding voxels.
For each subject I have run the first level model in spss and saved the 1000 most activated voxels for a given contrast as an roi (binary file).
Now with freesurfer I am trying to :
1) display the saved roi on the subject's surface (which has been obtained with recon all) - this step worked
2) align each subject's surface to fsaverage - this step gave me the problem
3) add all subjects together and create the map showing how many subjects activated each location

To do so I have done the following:

1) this first step worked fine and I could display the roi on my subject's surface
mri_vol2surf --mov ${DATA_DIR}/sub01/vois/1000vox_IPS.nii \
    --reg ${DATA_DIR}/sub01/fmri/register.dat \
    --hemi rh \
    --o ${DATA_DIR}/sub01/probmap/testIPS.rh.mgh \
    --projfrac ${thisprojfrac}

2) this step crashed
mri_surf2surf --hemi rh --s fsaverage --fwhm 3 --cortex \
    --sval ${DATA_DIR}/sub01/probmap/testIPS.rh.mgh --tval ${DATA_DIR}/sub01/probmap/testIPSfs.mgh

The error is:
Loading source data
ERROR: dimension inconsistency in source data
       Number of surface vertices = 163842
       Number of value vertices = 133030

Do you have any idea why?

3)Then I was planning to use fscalc and add all subjects together, is this correct?

If there is another/easier way to do the same thing, please let me know

Thanks in advance for your help,
Best

Elisa