In clinical studies, researchers measure and control what people are consuming. But because the federal government still classifies marijuana as a schedule 1 substance, meaning it is considered to have a high potential for abuse, researchers face tough restrictions on the products they can give study participants.

The National Institute on Drug Abuse Drug Supply Program provides marijuana for research purposes, Gruber said. But research-grade marijuana isn’t what people are consuming in their homes.

“We’ve discovered each of these diseases hits a different part of the language network,” said lead author Dr. M. Marsel Mesulam, director of Northwestern’s Mesulam Center for Cognitive Neurology and Alzheimer’s Disease.

“In some cases, the disease hits the area responsible for grammar, in others the area responsible for word comprehension. Each disease progresses at a different rate and has different implications for intervention.”