Marijuana smokers aged between 18 and 45 are wanted for a university study on brain activity.

The only catch is that cannabis users who sign up for the study will have to lay off the 'wacky backy' during the trial.

Auckland University scientist Dr Bruce Russell said the research will examine whether regular recreational cannabis use affects decision-making.

He'll be pitting students with little or no history of cannabis use, against those who've been smoking it at least twice a day.

For around $20 an hour students will have their brain activity monitored to compare activity associated with thought processes when they're sober.

"We're not giving anyone any drugs — we just want to see what the effects might be," said Russell.

Each participant would be connected to an electroencephalograph (EEG) by electrodes fitted to their scalp. Brain activity would be monitored while participants went through a series of tests on a touch-screen computer.

"It's a modified gambling task to try to differentiate the evaluation of risk versus reward," said Russell.

"We're looking to see if it [regular cannabis use] affects people's ability to judge risk or make them engage in slightly riskier behaviours than they might normally."

Russell said all participants would be given a toxicology test. For the study to be reliable, it was important participants were not under the influence of any illicit drug or alcohol.