This study actually shows that marijuana induces the growth of new brain cells and fights depression:

http://www.foodconsumer.org/cgi-bin/...iew.cgi/6/1682

This one (found on NORML's website) gives a full report on cannabis and the brain:
http://www.norml.org/index.cfm?Group_ID=6812

Exposing marijuana myths:
http://www.erowid.org/plants/cannabi...bis_myth.shtml
(Erowid is also a good source to check out pertaining to other info on cannabis as well).

Hope this can help you.