The 2011 Monitoring the Future Survey reported that there is an increase in American youth marijuana use and that there has been an attenuation of perceived risks associated with regular marijuana use . These trends highlight the importance of understanding the impact of marijuana on neural processing.Using fMRI Tapert et al.compared adolescent marijuana users and nonusers during a Go/No-Go task and found that users showed altered blood oxygen level dependent response during both Go and No-Gotrials even after 28 days of abstinence.This was interpreted as an increase in effort required to perform the task. Additionally,using a Stroop test,a measure of response inhibition,and fMRI,compared adult marijuana users and nonusers, with the users testing positive for recent marijuana use in a urine test. Consistent with,they found greater activation, in the users compared to nonusers,in the dorsolateral prefrontal cortex. In addition, they also found that users showed decreased activity in the anterior cingulate cortex.
These results were interpreted to suggest that the marijuana smokers used different cortical processes than nonusers to perform the task. In two more recent studies, results also illustrated that active marijuana users display greater levels of functional abnormalities than abstinent users in frontal, parietal and cerebellar brain regions as they performed other executive functioning tasks, including visuospatial working memory.Again, similar interpretations were suggested,including that marijuana users were required to recruit different neural pathways to perform the tasks and that exposure at a young age may increase the vulnerability to these effects. Despite these findings, further data is needed to more clearly specify and elaborate how early exposure to marijuana affects neural processing in young adult hood.An important requisite in this quest is a well controlled sample.The differences in neural activation in marijuana users are mostly due to delta-9-tetrahydrocannabinol ,marijuana’s most active psychoactive ingredient,which acts as a ligand for human cannabinoid receptors.The wide distribution of these receptors in the human brain,with particularly high densities in the cerebellum, parts of the basal ganglia, hippocampus, and many regions of the neocortex,poses great concern for the maintenance of a healthy ability to cognitively process information.Considering the neurodevelopment that occurs during adolescence and young adulthood, specifically, prefrontal cortex development and the subsequent advancement of executive functioning,it is clear that understanding this neural impact of marijuana in youth is imperative.
Further understanding this impact was the goal of the present study.The Ottawa Prenatal Prospective Study is an ongoing longitudinal investigation that was initiated in1978, with the primary objective of examining the effects of “soft” prenatal drug exposure on offspring. Children were followed from infancy to young adulthood where detailed information has been collected on their prenatal drug exposure, current and past drug use, cognitive/behavioral performance, and over 4000 lifestyle variables. Using this unique sample in combination with the powerful imaging technique, fMRI, and a well established Go/No Go task, the purpose of the present study was to determine if there was a significant relationship between brain activity and marijuana use and if this could be observed in young adults with relatively few years of exposure. Based on previous research where marijuana users and nonusers showed no differences in task performance,it was hypothesized that there would be no performance differences between groups for the present study. Despite this, marijuana users would require greater activation than controls in brain regions that typically demonstrate response inhibition in order to successfully perform the task, including the prefrontal cortex.
The most substantial differences in activation were found to be right lateralized in the premotor cortex and the middle frontal gyrus or dorsolateral prefrontal cortex.Response inhibition in healthy controls involves a distributed network that includes these areas as well as parietal areas.During response inhibition, the premotor cortex is involved in response competition and the preparatory process leading to correct initiation or suppression of movement . Given that there were non-significant behavioral differences in errors of commission between the two groups it is unlikely that this increased activation of the premotor cortex is related to increased motor responses in the users. Also,found that mostly left premotor cortex is involved during preparation to respond. Thus, the findings from the present study suggest that marijuana smokers may need to compensate by recruiting homologous contralateral areas in order to correctly initiate or suppress responses.