Generally speaking, the smaller a person's hippocampus -- a part of the brain associated with learning and memory -- the worse their memory will be. Researchers found, however, that for people who use both marijuana and tobacco, the smaller their hippocampi, the better their memory was during tests.
The inverse relationship surprised researchers, who note that while there has been a lot of research on the effects of nicotine and marijuana on the brain by themselves, there haven't been many to look at the combination.
"Approximately 70 percent of individuals who use marijuana also use tobacco," said Dr. Francesca Filbey, the Director of Cognitive Neuroscience of Addictive Behaviors at the Center for BrainHealth, in a press release. "Our findings exemplify why the effects of marijuana on the brain may not generalize to the vast majority of the marijuana using population, because most studies do not account for tobacco use."
Researchers compared four groups of participants in the study: people who smoked marijuana at least four times per week, people who smoked at least 10 cigarettes per day, people who smoked both and people who had used neither in the previous 3 months. The participants were all asked to complete neuropsychological tests and researchers used magnetic resonance imaging to examine their hippocampi.
The researchers were surprised to find that while smaller hippocampi tend to mean worse memory, participants who used both substances had smaller hippocampi but better memory. This effect, the researchers said, also was affected by how many cigarettes per day the person smoked. The groups who smoked only marijuana or tobacco did not show any link between hippocampal size and memory.
"Future studies need to address these compounding effects of substances," Filbey said. "The combined use of marijuana and tobacco is highly prevalent. For instance, a 'blunt' is wrapped in tobacco leaf. A 'spliff' is a joint rolled with tobacco. We really need to understand how the combined use changes the brain to really understand its effects on memory function and behavior."