How THC Alters Brain Networks: New Insights from Resting-State Neuroimaging
August 27, 2026
At the August 2026 CMCR Investigators’ Meeting, Jodi M. Gilman, Ph.D., Associate Professor of Psychiatry at Harvard Medical School and Director of Neuroscience at the Massachusetts General Hospital Center for Addiction Medicine, presented research examining how acute THC intoxication alters brain networks and whether these changes could help identify objective markers of impairment.
Dr. Gilman explained that blood and breath THC concentrations correlate poorly with impairment because THC rapidly moves into tissues and its effects vary with tolerance and other individual characteristics. Her research therefore focuses on what THC does to brain networks supporting cognition and behavior rather than how much THC is present in the body.
She described a double-blind, randomized crossover study of 128 regular cannabis users who received oral synthetic THC or placebo. Using functional near-infrared spectroscopy (fNIRS), researchers found that THC weakened connectivity between prefrontal regions, reduced normal anticorrelations, and made network connectivity more variable over time. Participants with greater disruption also reported feeling more intoxicated. THC additionally reduced spontaneous low-frequency brain activity.
Dr. Gilman also reviewed a whole-brain functional MRI study of chronic and occasional cannabis users. After vaporized THC, the brain was less likely to enter a highly integrated state associated with efficient communication across multiple networks. Greater disruption of this state was associated with poorer attention. Differences among chronic users during the placebo condition may reflect longer-term adaptation, although the findings do not establish lasting effects.
She cautioned that neuroimaging signals reflect both neuronal activity and physiological processes, including blood flow and vascular function, which THC can also affect. Future research must better distinguish these signals, account for individual differences, and determine whether brain alterations associated with chronic use normalize after cessation. In closing, Dr. Gilman suggested that portable technologies such as fNIRS may eventually help translate neuroimaging findings into practical, brain-based measures of THC impairment.
Jodi M. Gilman, Ph.D.
Associate Professor of Psychiatry
Harvard Medical School
Director of Neuroscience, Massachusetts General Hospital Center for Addiction Medicine
Dr. Gilman is an addiction neuroscientist whose research focuses on understanding how substance use affects brain development, cognition, and behavior across the lifespan, with a particular emphasis on cannabis and other commonly used substances. She leads an interdisciplinary research program that combines advanced neuroimaging methods, including MRI, MRI, PET, and near-infrared spectroscopy, with behavioral, cognitive, and clinical assessments to investigate the biological mechanims underlying substance use and addiction. Her work spans both observational and experimental research, from large-scale developmental studies examining risk factors or substance use to laboratory-based investigations of acute drug effects on brain function.