this study because they just legalized medical marijuana. gruber's goal is to study new patients who have never used cannabis. >> our goal was to look at these folks before they started using medical marijuana and then three months in, six months in, and then a year. primarily to look at frontal executive functions because that's really the question. >> that question -- how does medical marijuana impact brain function longer term? >> i want you to name the blocks of color for me as quickly as you can. >> before and after medicating, patients will take cognitive tests and here's what's revolutionary. they're going to have their brains scanned to chart the change. for the first time, we will see what your brain looks like on weed. >> nearly half the country now has medical marijuana laws on the books. this is happening and yet one of the most fundamental questions remains unanswered. >> right. >> that's what you're trying to do. >> i'd like to see what it looks