[Freesurfer] Volume maps unit in FreeView

Hi,

I have a very simple question.

I have cortical volume maps (averaged over 30 subjects). I can view those in FreeView using lh.pial overlaid with lh.mean.volume.mgh file. In overlay

configure option, I had to set the min as 0 and max as 4 to display the

average volume. Now the color bar is showing scale between 0 (min as blue color) and 4 (red) as maximum. I am not sure what does this 0 and 4 represent here, what are its units and how the actual volume values 20,000-40,000 mm^3 converted to this range between 0 and 4?

Similarly, for surface area, color bar range is between 0 and 0.9 when actual values range around 11000-14000 and for cortical thickness, color bar range is between 1 and 3 when actual values range around 2.1-3 mm.

Any help please.

Thanks.