Doesn't affect
brain function, it affects the health of the lungs.
And by smoking very potent pot, you can reduce the amount of smoke that enters your lungs by an incredibly amount, to the point where one hit physically can not cause permanent damage.
EDIT:
Here's a link to a medical article on this topic
http://www.webmd.com/mental-health/news/20030701/heavy-marijuana-use-doesnt-damage-brain
People misguidedly associate things they've heard about tobacco and associate it with marijuana. The fact is tobacco smoke does numerous things that just doesn't occur with marijuana smoke, in fact, cannabinoids (the psychoactive substances in marijuana) have been shown to do the
opposite of tobacco smoke in a few cases.
A few examples include
-slowing the growth of tumors,
-Anti-asthmatic, anti-rheumatic and anti-inflammatory effects
-Feels good, man.