Interrupting spells while on mounts??

[Spuukee]

How do you go about interrupting a spell while on horseback? I'm kind of new to the horse thing, and I can't figure out how to do it. I usually duck to interrupt spell, but I can't duck while on horseback.

[Majordeath]

Ctrl + left or right arrow will move you quickly enough to interrupt most spells.
Moving forward or back can be a bit to slow to interrupt.