The age of onset of cannabis use and executive function.

This thesis explored the association between cannabis use and executive function, with a specific focus on the age of onset of cannabis use. Previous research has provided some preliminary evidence that early onset cannabis use, namely cannabis initiation during the early teenage years or younger, is associated with greater cognitive deficits than later onset use. The first study within this thesis included a large battery of validated neuropsychological tasks administered to cannabis users, controls and tobacco using controls to compare executive function performance. This approach found several cognitive domains in which cannabis users performed at significantly lower levels than the two controls groups. Further analyses showed that the early onset cannabis users (16 years) when compared to the control groups. While the early onset hypothesis did not explain all of the deficits in the cannabis using groups there was a clear pattern for early onset users to be impaired at a greater level on some tests. In particular, it appeared that the cannabis-related deficits were more pronounced on tests which required visual scanning, set switching abilities and visuo-motor coordination. While the visuo-motor deficits were quite clear, it could not be determined if visual scanning deficits were explaining the performance deficits on the set switching task.Subsequently an eye-tracking study was conducted to investigate the age of onset of cannabis use effects on visual scanning and set switching processes. This study suggested that visual scanning was impaired while set switching processes remained intact in cannabis users. Furthermore, this study found some support of an age of onset effect; two of the dependent variables showed evidence of greater impairments among early onset cannabis users relative to the other groups.In addition this thesis includes a large scale survey which assessed lifestyle predictors of the age of onset of cannabis use. Two lifestyle factors were highly associated with executive function abilities - other drug use and educational achievement - were predictors of the age of onset of cannabis use. As the two quasi-experimental studies controlled for these two variables, in addition to other confounding variables, it is unlikely they were explaining the pattern of deficits reported.Based on data presented here and supported by existing multidisciplinary research, it is argued that the relationship between cannabis use and cognition reported in this thesis is causal. While cannabis may have a causal impact on cognitive performance, such as visual search and visuo-motor coordination, the cannabis users still performed at a "normal" level. The severity of cognitive impairments does appear to be mediated by early onset cannabis use, however not all cognitive processes appear to be at risk for these greater impairments. Findings may indicate a developmental vulnerability to cannabis use.