The longitudinal neuro-imaging study, published in the March 2014 issue of Addiction Biology, was investigating the relationship between substance use (alcohol, cannabis, nicotine, and illegal psychotropic drugs) and working-memory network function over time in heavy cannabis users, and in controls.
Forty-nine participants performed an n-back working-memory task at baseline, and at a three-year follow-up. At follow-up, there were 22 current heavy marijuana users, four abstinent heavy users and 23 non-cannabis-using controls. Tensor-independent component analysis (Tensor-ICA) was used to look at individual differences in working-memory network functionality over time.
Within the group of marijuana users, “cannabis-related problems” remained stable, whereas alcohol-related problems, nicotine dependence and illegal psychotropic drug use all increased over time.
At both measurements, both the initial measurement and the three-year follow-up, behavioral performance and network functionality during the n-back task did not differ between heavy users and controls.
Marijuana’s effect on short-term memory is well-known; it makes many users forgetful while they are actively under its influence. Whether cannabis affects memory beyond short-term once the high wears off — that is, whether it affects working and long-term memory — has been a subject of hot contention.
Working memory, while not completely distinct from short-term memory, is a term used to refer to memory as it is used to plan and carry out behavior, according to Nelson Cowan in the journal Progress in Brain Research. We rely on our working memory to, for instance, remember the partial results while solving a math problem in our heads, or to combine the parts of a long rhetorical argument — or to bake a cake without adding the same ingredient twice.
The term became much more prevalent after researchers demonstrated in 1974 that the neat categories of short-term and long-term memory simply didn’t cover all the bases when it comes to temporary memory.
Since deficient “executive functions” are seen as playing an important role in the development of addiction, working-memory could be a powerful predictor of the course of drug use, according to the investigators — and the chronic use of some substances, including alcohol, may also impair working memory. This study, therefore, aimed to find if the same is true of cannabis, and it seems not to be.
“These results suggest that sustained moderate to heavy levels of cannabis, nicotine, alcohol and illegal psychotropic substance use do not change working-memory network functionality,” the researchers wrote. “Moreover, baseline network functionality did not predict cannabis use and related problems three years later, warranting longitudinal studies in more chronic or dependent cannabis users.”