Study: fMRI shows disparate effects of marijuana

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Different ingredients in marijuana appear to affect regions of the brain differently during brain processing functions involving responses to certain visual stimuli and tasks, according to a study published in the January issue of Archives of General Psychiatry.
Sagnik Bhattacharyya, MBBS, MD, PhD, at the Institute of Psychiatry at King’s College in London, and colleagues studied 15 healthy men, who were occasional marijuana users to examine the effects of ?9-tetrahydrocannabinol (?9-THC) and cannabidiol (CBD) on regional brain function during salience processing, which is how people perceive things around them. Prior to the study, researchers had not yet investigated whether ?9-THC modulates salience processing in humans or evaluated its psychotogenic effects.

The authors used functional MRI ( fMRI) to study each participant on three occasions after administration of ?9-THC, CBD or placebo. Study participants performed a visual oddball task of pressing buttons according to the direction of on-screen arrows as a measure of attentional salience processing.

“Pairwise comparisons revealed that ?9-THC significantly increased the severity of psychotic symptoms compared with placebo and CBD, whereas there was no significant difference between the CBD and placebo conditions,” the authors wrote.

?9-THC had a greater effect than placebo on reaction time to nonsalient compared with salient stimuli. This was associated with modulation of both prefrontal and striatal function by ?9-THC, augmenting activation in the former region and attenuating it in the latter.

“Moreover, in the present study, the magnitude of ?9-THC’s effect on response times to nonsalient stimuli was correlated with its effect on activation in the right caudate, the region where the physiological effect of ?9-THC was linked to its induction of psychotic symptoms,” wrote Bhattacharyya et al.

The findings led the researchers to note that “collectively, these observations suggest that ?9-THC may increase the aberrant attribution of salience and induce psychotic symptoms through its effects on the striatum and lateral prefrontal cortex … These effects may contribute to the effects of cannabis on psychotic symptoms and on the risk of psychotic disorders.”

When the effects of CBD were contrasted with ?9-THC and placebo with respect to the visual task there was a “significant effect” in the left caudate with CBD augmenting the response and ?9-THC attenuating it.

“These effects suggest that CBD may also influence the effect of cannabis use on salience processing–and hence psychotic symptoms–by having an opposite effect, enhancing the appropriate response to salient stimuli,” the authors wrote.

Bhattacharyya and colleagues concluded, “These data provide the first evidence, to our knowledge, that the effects of cannabis on psychosis may be mediated by influencing the neural substrate of attentional salience processing. They also provide experimental support for the salience model of psychosis, which proposes that psychotic symptoms develop through the inappropriate attribution of salience to nonsalient stimuli.”