>> simple tasks become very frustrating, like mixing paint, and then just sort of get into this state of, you know -- >> and why that happens is exactly what columbia university neuroscientist carl hart is investigating. >> exhale. >> research subjects in his lab smoke marijuana and then take a variety of cognitive tests. >> the affects will be disruption in memory, disruptions in inhibitory control. they will become slower at cognitive functioning, a wide range of things. these affects are temporary, but they're pretty pronounced, and they are clear. >> and it's slowly becoming clear to scientists what part of the brain is most affected. it's the prefrontal cortex.