You may always play misdirection targeting a cabal therapy or a cranial extraction.
However, if the card is written "target opponent", you have to choose a valid opponent for the one who cast the spell...
If you're playing 1 vs 1, you must choose yourself as an opponent

*** Don't remember if cranial and therapy have "taget opponent" or "target player"... ***
If one of this spell has "target player" and you misdirect it on your opponent, it's always his choice for the card: he may always choose a card he doesn't play
