Hi Oori,
That can be found in the "Display Options" menu of any
image window (click on 'Disp' in the lower left corner).
What you describe is a projection +/- 5 slabs.
a. click 'Disp' in the z-plane image window
b. set 'Project' to Maximum (or whatever)
c. set 'Slab' to 5 (so +/- 5 slices)
You mentioned doing this to functional data, however
this is an operation on the Underlay. Hopefully that is
what you mean.
- rick