Cannabis has well established effects on cognitive processing but the neural basis of these is unclear. We used functional neuroimaging to investigate this, focusing on tasks that engaged verbal memory and response inhibition.

Subjects were 15 healthy males who had used cannabis < 25 times in their lifetime. Each subject was studied on 3 occasions, and was given either THC, CBD or placebo 1 hour prior to scanning, in a double-blind design. The order of drug administration was randomised and there was 1 month between each scanning session. During each session, images were acquired on a 1.5T GE camera while subjects performed a verbal paired associates memory task and a Go/No Go task. The modulatory effects of THC and CBD relative to placebo were examined by comparing activation during each task.

During the encoding phase of the memory task THC attenuated activation in the left temporal cortex compared to placebo. During the go-no go task, THC attenuated activation in the right inferior frontal cortex. Neither of these effects were attributable to differences in behavioural performance, sedation, or intoxication. The severity of psychotic symptoms provoked by THC was a function of its effect on right inferior frontal activation during response inhibition.

The effects of cannabis on verbal memory and motor control may be mediated through the influence of THC on left temporal and right inferior frontal activity, respectively. The induction of psychotic symptoms by cannabis may reflect an effect of THC on right inferior frontal activity.