Captain Robert April wrote:
Doohan also often said that if he did a truly authentic Scottish accent, you wouldn't have understood a bloody word Scotty said (much like the bloke from Aberdeen who he based the accent on in the first place). He toned it down out of necessity.
As for Craig Ferguson, he often jokes about how his family back in Scotland accuse him of having lost his accent.
Craig's other Scots gag is saying that the accent Doohan did (and others attempting a Scots accent) sounds like a Pakistani.
I've noticed Craig's accent gets thicker when he talks to other Scots, like Karen Gillan.