In a comprehensive analysis of previous research, scientists have found significant differences in the brain’s white matter between regular cannabis users and non-users. These findings, published in Frontiers in Neuroimaging, offer a crucial understanding of how cannabis use might impact brain function.
White matter, found in the deeper tissues of the brain, consists of millions of nerve fibers. These fibers act like communication cables, connecting different parts of the brain and allowing them to communicate with each other. The ‘microstructure’ of white matter refers to the intricate organization and quality of these nerve fibers.
Changes or disruptions in this microstructure can affect how well brain regions communicate, potentially impacting everything from cognitive abilities to mental health. Given the widespread use of cannabis globally, researchers are keenly interested in understanding how regular cannabis use might influence this crucial aspect of brain function.
“Many youths experiment with using cannabis, but it is unknown if and how this can affect brain development,” explained PhD candidate Emily Robinson (@em_robinson_12) and Associate Professor Valentina Lorenzetti (@vale_addiction), the deputy director of the Healthy Brain and Mind Research Centre at Australian Catholic University.
“It is part of the mission of the Neuroscience of Addiction and Mental Health Program, to unpack the origins of substance use behavior and addiction, so we decided to launch a few projects in this area. Our PhD student Emily has always been interested in how environmental variables such as substance use influence the neurodevelopment of white matter, which supports efficient neural communication; and she took the challenge of leading the review.”
For their review, the researchers sifted through 2,712 studies. They distilled this down to 30 studies that met their stringent criteria. The studies had to be in English, involve human samples, use diffusion-MRI to assess white matter integrity, and compare cannabis users with a control group. The researchers excluded studies involving regular use of other illicit substances, major medical or mental health conditions, assessments during acute cannabis intoxication, non-peer-reviewed or non-empirical sources, other neuroimaging techniques, and outcomes unrelated to white matter. These studies collectively included 2,898 participants.
The researchers found that cannabis users generally showed lower integrity in the superior longitudinal fasciculus, a key nerve bundle associated with executive functions like planning and problem-solving. Additionally, the corpus callosum, which connects the brain’s two hemispheres, showed higher mean diffusivity in cannabis users, indicating possible alterations in inter-hemispheric communication. These changes were more pronounced in those who started using cannabis at an earlier age and those who used it for longer periods.
“Using cannabis during youth can be associated with changes in the integrity of what we call white matter ‘microstructure’: these are like highways that connect many different parts of the brain, even those that are very distant from each other, and which support very different cognitive functions and emotion,” Robinson and Lorenzetti told PsyPost. “Starting to use cannabis at a young age, or over a longer period of time can intensify the changes, but this is not conclusive and the scientific community are still trying to confirm this.”
The research, however, presents a complex picture. While many studies indicated lower white matter integrity in cannabis users, some findings were inconsistent, and a few studies did not find significant differences. This variability underscores the complexity of the brain and how individual factors like genetics, environment, and lifestyle choices can influence the impact of cannabis on brain structure.
“Different labs measure how youth use cannabis in many ways that cannot be compared with one another,” Robinson and Lorenzetti explained. “It’s like comparing “pears and apples”. For example, some studies measure “occasions” when cannabis is used without clarifying over how many days and what period of time.”
“Until scientists measure cannabis in more consistent ways, we will not have data to understand if using cannabis more often, for long or more heavily is more detrimental for young people. Our lab has launched some initiatives to overcome this such as the National Institute of Health endorsed Standard THC Unit and the iCannToolkit.”
This review opens up several avenues for future research. One key area is the need for longitudinal studies that track individuals over time to understand how cannabis use affects the brain’s white matter from before use begins through various stages of life. Such studies could help determine whether the observed changes are a direct result of cannabis use or if they predate drug use.
Additionally, there’s a call for more standardized methods of measuring and reporting cannabis use in scientific studies. This standardization would make it easier to compare results across different studies and build a more comprehensive understanding of cannabis’s impact on the brain.
Lastly, the review suggests exploring newer and more sophisticated imaging techniques to gain a deeper understanding of the subtle changes that cannabis might induce in the brain’s white matter. These advanced methods could provide clearer insights into the specific ways cannabis use alters brain structure and function.
“It is still a mystery at what age these changes begin to appear and who are the youth who will go on using through adulthood and who will show these brain changes,” Robinson and Lorenzetti noted. “Also, we are yet to uncover if some brain changes predated the onset of cannabis use and represent a neurobiological vulnerability.
“A major caveat is the way in which white matter microstructure is quantified – a technique called diffusion tensor imaging, has some major limitations that prevent identifying changes with precision. New tools have been developed that measure brain changes with greater precision, such as fixel based analysis. We also identified that no studies were undertaken prior to the onset of cannabis use, and these are both things we hope to address in upcoming research.”
The study, “Measuring white matter microstructure in 1,457 cannabis users and 1,441 controls: A systematic review of diffusion-weighted MRI studies“, was authored by Emily Anne Robinson, John Gleeson, Arush Honnedevasthana Arun, Adam Clemente, Alexandra Gaillard, Maria Gloria Rossetti, Paolo Brambilla, Marcella Bellani, Camilla Crisanti, H. Valerie Curran, and Valentina Lorenzetti.
