Draulio de Araujo of the Brain Institute at the Federal University of Rio Grande do Norte in Natal, Brazil, and colleagues recruited 10 frequent users of the brew – called . They asked the volunteers to look at images of people or animals while their brains were scanned using , then asked the volunteers to close their eyes and imagine they were still viewing the image. Unsurprisingly, the researchers found that neural activity in the primary visual cortex dropped off when volunteers imagined seeing the image rather than actually viewing it.

But when the team then gave the volunteers a dose of ayahuasca and repeated the experiment, they found that the level of activity in the primary visual cortex was virtually indistinguishable when the volunteers were really viewing an image and when they were imagining it. This means visions seen have a real, neurological basis, says de Araujo – they are not made up or imagined.

, head of the brain imaging unit at King’s College London, says the study’s statistics appear to indicate something relatively robust. However, he says it’s difficult to pin down whether the eyes-closed responses on the drug are quantitatively the same as normal, eyes-open neural activity. “Functional MRI is not a one-to-one mapping of cerebral activity. If it were, things would be easier,” he says.

of Imperial College London has done similar fMRI work using the “magic mushroom” hallucinogen psilocybin. He says the results also have practical implications, such as for the application of psychedelics in psychotherapy.

Ayahuasca may also find its way into the psychiatrist’s drug kit. The pharmacology of tallies with the way some conventional drugs work; because of this, researchers are interested in ayahuasca’s potential for treating addiction, depression or conditions such as post-traumatic stress disorder. One of the brew’s two ingredients is the vine Banisteriopsis caapi, which contains chemicals that act as – a major class of antidepressant drugs. The other ingredient is the shrub Psychotria viridis: it contains the powerful hallucinogen DMT (dimethyltryptamine), which acts on the mood-altering , the target of antidepressants such as Prozac.

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Psychedelic drug users throughout the ages have described their experiences as mind-expanding. They might be surprised, therefore, to hear that psilocybin – the active ingredient in magic mushrooms – actually decreases blood flow as well as connectivity between important areas of the brain that control perception and cognition.

The same areas can be overactive in people who suffer from depression, making the drug a potential treatment option for the condition.

The study is the first time that psilocybin’s effects have been measured with fMRI, and the first experiment involving a hallucinogenic drug and human participants in the UK for decades.

at Imperial College London and colleagues recruited 30 volunteers who agreed to be injected with psilocybin and have their brain scanned using two types of fMRI.

Half of the volunteers had their blood flow measured during the resulting trip; the rest underwent a scan that measured connectivity between different regions of the brain.

Low flow

Less blood flow was seen in the brain regions known as the thalamus, the posterior cingulate and the medial prefrontal cortex. “Seeing a decrease was surprising. We thought profound experience equalled more activity, but this formula is clearly too simplistic,” says Carhart-Harris. “We didn’t see an increase in any regions,” he says.

Decreases in connectivity were also observed, such as between the hippocampus and the posterior cingulate and medial prefrontal cortex.

“Under psilocybin you see a relative decrease in ‘talk’ between the hippocampus and these cortical hub regions,” says Carhart-Harris. “Changes in function in the posterior cingulate in particular are associated with changes in consciousness.”

Mood swing

Psilocybin has a similar chemical structure to serotonin – a hormone involved in regulating mood – and therefore binds to serotonin receptors on nerve cells in the brain. The drug may have therapeutic potential because the serotonin system in nerves is also a target for existing antidepressants.

A study earlier this year by at the University of California, Los Angeles, showed that people with end-stage cancer had significantly less anxiety and better mood after receiving psilocybin ().

, who works in a similar field at the Psychiatric University Hospital Zurich, Switzerland, says that the immediate effects of psilocybin are not as important for clinical benefit as the longer-term effects. That’s because psilocybin increases the expression of genes and signalling proteins associated with nerve growth and connectivity, he says: “We think that the antidepressant effects of psilocybin may be due to a possible increase of factors that activate long-term neuroplasticity.”

Carhart-Harris presented his work at the conference at the University of Kent in Canterbury, UK, this week

These are the findings of a major new study by British transport researchers. The unpublished research, seen exclusively by żìĂš¶ÌÊÓÆ”, stops well short of condoning driving under the influence of even small amounts of cannabis. But in a week which has seen renewed debate in Britain surrounding the criminalisation of cannabis, it throws an uncomfortable spotlight on a problem confronting governments everywhere – how to deter the growing numbers of cannabis users from “dope driving”.

At present there is no accurate test that can reveal whether a driver has taken cannabis before driving, and developing one will not be easy. But even when this problem is cracked, another will remain – where to set the safety threshold for smoking cannabis.

Advocates of zero tolerance say there should be penalties for drivers caught with any amount of recently smoked cannabis in their body. The new research suggests that would only be credible if governments also adopted zero tolerance on drink driving.

Middle of the road

The new study was undertaken by the Transport Research Laboratory in Crowthorne, Berkshire, and confirms the results of a preliminary study more than a year ago. Researchers at the TRL, led by Barry Sexton, gave 15 volunteers doses of cannabis or alcohol, or a combination of both, before letting them loose on an array of psychomotor tests and a sophisticated driving simulator.

The volunteers were given either enough alcohol to raise alcohol levels in the blood to 50 milligrams per 100 millilitres – about 60 per cent of Britain’s legal limit of 80 mg/100 ml – or a specially prepared marijuana joint designed to deliver the same high typically experienced by smokers.

In the study, cannabis significantly affected only one criterion, known as tracking ability. Volunteers found it more difficult to hold a constant speed and follow the middle of the road accurately while driving around a figure-of-eight loop. The TRL researchers point out in their draft report that this test requires drivers to hold their concentration for a short time, a task which is particularly badly affected by the intoxicating effects of cannabis.

Cautious driving

However, volunteers drinking the equivalent of a glass of wine fared worse than those who had smoked a joint. Those who were given both alcohol and cannabis performed worse still, reinforcing the idea that alcohol has a cumulative effect when taken with other drugs.

But the study also found that drivers on cannabis tended to be aware of their intoxicated state, and drove more cautiously to compensate. Indeed, doped-up volunteers often rated themselves as being more impaired than police surgeons brought in to evaluate their sobriety.

Surprisingly, drinking alcohol didn’t offset this cautious behaviour, opening up the unproven possibility that a driver who is moderately drunk might be better off under some conditions if they had also smoked.

This cautious behaviour is in line with findings by other researchers. “Whereas alcohol promotes risk taking like fast speeds and close following, cannabis promotes conservative driving, but may cause attention problems and misperceptions of time,” says Nicholas Ward, technical adviser to the Immortal project – a three-year European Union trial designed to quantify the crash risk drivers face after taking various drugs and medicines.