How do spells decide which intro to play?

As you know, FF6 has five casting intros: Black, White/Effect, Lore, Blitz, and Swordtech.
But... I can't figure out how the game determines which one to use.  Is it strictly based on which command calls it?  Is it by spell index?  I couldn't find references to the intros in Banks C1 or C2.

It's always bothered me that Effect magic and White magic had the same intro.  I'm looking to set Effect magic to use Lore's intro instead, if such a thing is possible.