Contrary to stoner stereotypes, marijuana usage has actually been shown to have some positive mental effects, particularly in terms of increasing creativity. Even though people's short term memories tend to function worse when high, people get better at tests requiring them to come up with new ideas.
One study tested participants on their ability to come up with different words related to a concept, and found that using cannabis allowed people to come up with a greater range of related concepts, seeming "to make the brain better at detecting those remote associations that lead to radically new ideas," according to Wired.
Other researchers have found that some participants improve their "verbal fluency," their ability to come up with different words, while using marijuana.
Part of this increased creative ability may come from the release of dopamine in the brain, lessening inhibitions and allowing people to feel more relaxed, giving the brain the ability to perceive things differently.
Other research has shown that marijuana stimulates areas of the brain responsible for: pleasure, memory, learning, sensation, sensation of time and space, coordination, movement, appetite, and other drives.