afni.nimh.nih.gov

Yes. The simplest way is to leave the "Pos" button at the bottom of the colorscale UN-checked, and then do this:

a) right click on the label at the top of the threshold slider to get a popup menu
b) about 2/3 of the way down is a chooser labeled "Sign"
c) by default, this chooser is set to "Pos & Neg"
d) change it to "Neg only" to see only voxels whose threshold is below the negative of the slider value

I hope this is what you want.