What comes as little surprise to those familiar with the ECS (endocannabinoid system), a recent Harvard medical study showed that smoking cannabis over a three month period improved cognitive performance. What???
Well, cannabis contains cannabinoids. Your body happens to produce similar compounds called 'endocannabinoids'. So, we know that endocannabinoids promote embryonic and adult hippocampus neurogenesis...in other words, they create new brain cells. So, when your body produces certain endocannabinoids through certain methods contained at this link, it helps to promote the birth of new brain cells.
So, since cannabis contains cannabinoid compounds that work in identical fashion to the same endocannabinoids produced by your body, it would make sense to have cognitive improvement, all other things being equal.
However, there is a major case to be made for the 'entourage effect' made famous by Israeli scientist Dr Mechoulam in 1998. There are two main chemicals that make up approximately 98% of the cannabis plant, THC and CBD. However, there are also approximately 90 other cannabinoid like compounds that act synergisticly with the other cannabinoids to produce varying effects. When the cannabinoids are used synergisticly, we get an 'entourage effect' of more effective medication.
For example, another cannabinoid called CBC (cannabichromene), has recently been shown to increase the viability of adult neural stem progenitor cells essential for brain plasticity and suggestive of neurogenesis.
As biotech companies get better at varying cannabinoid combination for treatments, we will see more focus on these smaller compounds and the role they play with THC and CBD.