Hi-

You can turn off the "auto-symmetrized" intensity in the SUMA controller.

Beneath the grid of values where you are entering Min/Max intensity, and below the "Col" and "Cmp" entries are a few briefly named options with squares to toggle behavior on/off. Unselect the square to the left of "sym I"-- that is what enforces the symmetric ranginess. (To see more details, click on the "BHelp" button at the upper-left of the Control Panel, and then click on that option name again to see the full "button help" about it.

It is the bottom-most item circled in magenta here:
[afni.nimh.nih.gov]
if that helps.

--pt