Okay, that is entirely fictional. I personally do not feel that Cannabis causes in sort of mental-instability or dysfunction, but that under the influence of a subtle, yet powerful psychedelic, which marijuana is, you are capable of coercing your mind to believe anything - even that you are mentally insane. The side-effects of depersonalization and derealization accumulated with that, and likely used by the user as evidence of his own slipping mental-state, do not help either.

Here is an older, but contradictory article:
www.cmcr.ucsd.edu/geninfo/Reuters_062703.pdf

And the part about the brain having a part of it, used solely to keep out inappropriate thoughts and behaviour, and the fact that pot inhibits its function - well, I'd call that "mind-expanding" and obviously a side-effect of "mind-manifesting," or psychedelic drug.