Small sample sizes in prior studies (generally N<100) and varied definitions of cannabis exposure may have contributed to these inconsistent findings. Additionally, studies have implied that cannabis causes volumetric alterations, despite typically not controlling for potential confounding effects of shared predispositional factors (e.g. genes/rearing environment that contribute to both volumetric differences and cannabis use). For instance, Gilman et al. (2014) compared recreational cannabis users (N=20) with matched non-users (N=20) and posited, based on observed dose-dependent relationships, that larger left ventral striatal (VS) grey matter density was likely a consequence of cannabis use7. However, such cross-sectional case-control designs cannot account for the possibility that volumetric variations might predate cannabis use and/or might relate to cannabis use via predispositional factors, even in a dose-dependent manner.
