afni.nimh.nih.gov

You can control the display contrast scaling by right-clicking on the color bar for the AFNI underlay in the display viewer and then "Choose Display Range." Set the bottom to 0 and the top to a fixed intensity level that fits the data.

You can also set the AFNI environment variable, AFNI_IMAGE_GLOBALRANGE, to YES to scale the display from the minimum voxel to the maximum voxel in the dataset. Note this setting changes the default from the usual 2%-98% or min to max of current slice scaling.