Cannabis can affect your short term memory. Maybe thats why some people fall under the myth that it makes you dumber.
Personal experience tells me that it makes you smarter.

Cannabis enhances your nervous system, increases blood flow to the brain. Studies on IQ show that doing something differently will make you smarter. Now...when youre high everything seems different. To me this indicates a good possibility that cannabis can be used as a tool to learn beyond the constricts of a sober mind.