horizontal scroll on normal mice

I'm desperately looking for an option to map certain mouse-buttons (6 and 7 i.e. the mouse wheel to the left/right on a Logitech mouse) to horizontal scrolling. Does anybody know how to do that? SHIFT-scroll does not work and I didn't find any predefined action for horizontal scrolling. Nor any hint in the documentation.

You could assign the left/right arrows, but it works only smooth if you don't see your curser in a text box.