People try and run away from things and to forget, but with psychedelic drugs they're forced to confront and really look at themselves," explains Dr Robin Carhart-Harris, from Imperial College London. The drugs Carhart-Harris is referring to are hallucinogens such as magic mushrooms -- specifically the active chemical inside them, psilocybin. Carhart-Harris scanned the brains of 30 healthy volunteers after they had been injected with psilocybin and found the more primitive regions of the brain associated with emotional thinking became more active and the brain's "default mode network," associated with high-level thinking, self-consciousness and introspection, was disjointed and less active. "We know that a number of mental illnesses, such as OCD and depression, are associated with excessive connectivity of the brain, and the default mode network becomes over-connected," says David Nutt, professor of neuropsychopharmacology, who leads the Imperial College team. The over-connectivity Nutt describes causes depressed people to become locked into rumination and concentrate excessively on negative thoughts about themselves. Depression is estimated to affect more than 350 million people around the world. The current pharmaceutical approach to treatment is with selective serotonin re-uptake inhibitors (SSRIs), such as Prozac. But SSRIs ... are generally prescribed for long periods of time to maintain their effect. Nutt thinks psilocybin could be a game-changer, used as part of a therapeutic package ... to treat people within just one or two doses of treatment.