Laura Spinney: Science sniffs out secrets of aromatherapy

COMMENT

Aromatherapy is the most widely used complementary therapy in Britain's National Health Service, and doctors use it most often for treating dementia. It is believed to bring benefits to elderly patients who have difficulty interacting verbally, and to whom conventional medicine has little to offer. But last year, a review of healthcare databases found almost no evidence that aromatherapy was effective in the treatment of dementia. The only trial included in the review that seemed to show a benefit had flaws in the way it was done.

And a study in 1999 suggested that the use of aromatherapy for people with Alzheimer's and related dementias should be "abandoned or reassessed" because of the degeneration of the brain's olfactory bulb and the accompanying loss of sense of smell in such patients.

Other findings suggest aromatherapy works only on people who believe it will work.

Yet, touchy-feely placebo or not, aromatherapy is as popular as ever.

In January, French newspaper Le Monde reported a prison hospital in Fresnes, near Orly airport, had given its blessing to aromatherapist Marie-Therese Esneault, whose attempts to stimulate pleasant memories in mentally ill patients had led to dramatic improvements in their behaviour.

It is almost as if doctors, patients and therapists know that in smell they have a powerful means of manipulating emotion and memory, but are using it blindly.

But perhaps not for much longer, because researchers are now revealing links between the sense of smell, memory and emotion that could suggest far more precise targets for therapy.

Studies have consistently shown that odours elicit emotional memories far more readily than other sensory cues.

And this year, Rachel Herz, of Brown University in Providence, Rhode Island, and colleagues scanned the brains of five women while they either looked at a photo of a bottle of perfume that evoked a pleasant memory for them, or smelled that perfume.

One woman remembered how as a child living in Paris she would watch with excitement as her mother dressed to go out and sprayed herself with that perfume.

The women described the perfume as far more evocative than the photo, and Herz and co-workers found that the scent activated the amygdala and other brain regions associated with emotion processing far more strongly than the photograph.

But the interesting thing was that the memory was no better recalled by the odour than by the picture.

"People don't remember any more detail or with any more clarity when the memory is recalled with an odour," Herz says.

"However, with the odour, you have this intense emotional feeling that's really visceral."

That's hardly surprising, she thinks, because of the way the brain has evolved.

"The way I like to think about it is that emotion and olfaction are essentially the same thing.

"The part of the brain that controls emotion grew out of the part of the brain that controls smell."

That, she says, probably explains why memories for odours that are associated with intense emotions are so strongly entrenched in us, because smell was initially a survival skill - and indication of what to approach or to avoid.

Eric Vermetten, a psychiatrist at the University of Utrecht in the Netherlands, says doctors have long known about the power of smells to act as traumatic reminders.

But the evidence has been largely anecdotal.

Last year, at Emory University in Atlanta, Georgia, he and others set out to document it by describing three cases of post-traumatic stress disorder in which patients reported either that a certain smell triggered their flashbacks, or that a smell was a feature of the flashback.

The researchers concluded that odours could be used in exposure therapy, or for reconditioning patients' fear responses. When Vermetten presented his findings at a conference, doctors in the audience told him how they had turned this association around and put it to good use.

Stress disorder patients often have group therapy, but the therapy can expose them to traumatic reminders.

"Some clinicians put a strip of vanilla or a strong, pleasant, everyday odorant such as coffee under their patients' noses, so that they have this continuous olfactory stimulation," says Vermetten.

These patients seem to be better protected against flashbacks.

Nobody knows what's happening in the brain, says Vermetten, but it's possible that the neural pathways by which the odour elicits the pleasant, everyday memory override the fear-conditioned neural pathways that respond to verbal cues.

To Rachel Herz, the therapeutic potential of odours could lie in their unreliability.

She has shown with her perfume-bottle experiment that smells don't guarantee any better recall, even if the memories they elicit feel more real.

And there's plenty of research to show that our noses can be tricked.

In 2001, for instance, Gil Morrot, of the National Institute for Agronomic Research in Montpellier, tricked 54 wine students by secretly colouring a white wine with an odourless red dye just before they were asked to describe the odours of a range of red and white wines.

The students described the coloured wine using terms typically reserved for red wines.

And their descriptions showed they were influenced by the wine's appearance rather than its smell.

Smell, the researchers concluded, cannot be separated from the other senses. Last July, Jay Gottfried and Ray Dolan of the Wellcome Department of Imaging Neuroscience in London took that research a step further when they tested people's response times in naming an odour when shown either an image was associated with the odour or one that was not.

So they asked subjects to sniff vanilla and simultaneously showed them a picture of ice cream or of cheese, while scanning their brains.

Subjects named the smells more quickly when the picture showed something related to them, and when that happened, a brain structure called the hippocampus was strongly activated.

The researchers' interpretation was that the hippocampus plays a role in integrating information from the senses - information that the brain then uses to decide what it is perceiving.

In December, Herz showed that this kind of hippocampus trickery can also elicit dramatically different emotional responses.

She asked people to sniff jars that contained ambiguous odours. One odour was patchouli, which she called either "musty basement" or "incense"; another was a chemical combination she described either as "parmesan cheese" or "vomit".

In both cases, people reported pleasure or disgust depending on which label they saw. Not all smells detectable by humans possess this ambiguous quality.

But the susceptibility of olfactory perception to distortion by visual or verbal information would seem to suggest an obvious therapeutic intervention, says Herz.

Consider a rape victim who suffers a traumatic flashback each time she smells the aftershave worn by the rapist - a popular aftershave worn by millions of men.

If she could be persuaded by simple exposure to a mislabelled bottle that the smell was not aftershave but a brand of, say, cleaning fluid, it might be possible to recondition her neural pathways and break that association.

- INDEPENDENT