(?)Marijuana causing physical change in brain.

Recently a friend was telling me about a video he was watching. He claims that the video showed evidence that marijuana causes the thickness of the piece of flesh seperating the two halves of the brain to increase in size up to 400x. (In heavy users.) He claims that this lengthens the time it takes for the two halves of your brain to "communicate" and this causes permenant memory damage etc.

I don't know much about biology and therefore can not provide enough proof (For him anyway.) to convince this friend that this is untrue. Common sense and my own knowledge tells me that this would be impossible and if it were true it would have be common knowlegde.

Any info from someone more well versed in this subject would be much appreciated.

Thanks to all,
Anon