Cannabis-induced alterations in brain activation during a test of information processing speed in patients with MS

OBJECTIVE: The objective of this article is to determine the functional brain correlates of information processing speed in multiple sclerosis (MS) patients who smoke cannabis and those who are drug naïve. METHODS: Two neurologically and demographically matched samples of MS patients were enrolled, those who smoked cannabis daily (n = 20) and those who were cannabis naïve (n = 19). All participants completed the Brief Repeatable Battery of Neuropsychological Tests and underwent fMRI testing during which they were administered a modified version of the Symbol Digit Modalities Test (mSDMT). RESULTS: The cannabis group responded slower in nine of 11 blocks of the mSDMT (p < 0.001), showing a trend toward a slower response time (p < 0.08), but did not differ in the accuracy of response (p < 0.18). Both groups displayed activation in a prefrontal cortex-parietal network associated with information processing speed. When compared to the cannabis-naïve group, cannabis users showed less activation in the right (p = 0.009) and left (p = 0.001) thalami and increased activation in the anterior cingulate (p = 0.006). CONCLUSION: Regular cannabis use in MS patients is associated with slower information processing speed and a pattern of cerebral activity that differs from cannabis-naïve individuals, most notably in a bilateral reduction of thalamic activity.