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When she was 14, as Nicole Townsend agonized about the sort of girl she was, and how she looked compared to her friends, what began to take hold was a feeling that she didn't like herself very much. She figured losing a few kilograms would take her closer to perfection, but when that didn't work she set about losing more. By last Christmas, now 18, her weight had plunged from 55 kg to 32 kg. Racked by headaches and too weak to leave the house, she was often irrational yet also cunning in the ways she foiled her mother's attempts to make her eat. Because the girl she saw in the mirror still looked fat in the hips, thighs and stomach, she ignored her family's pleadings that she was skeletal. "I didn't believe them," she says. "That's not what I was seeing."

The anorexic patient's inability to see her body as it is has long fascinated researchers. While other manifestations of the illness (an obsession with weight and food, an intense fear of being fat) can be more readily traced to psychological distress, the anorexic's distorted perception of herself has suggested to many a biological abnormality — that something is amiss in the brain itself.

A study by two Sydney researchers, Perminder Sachdev and Naresh Mondraty, has now cast more light on this idea. Using magnetic resonance imaging (MRI) equipment, the pair detected marked differences in brain activity between anorexic women and healthy controls when the two groups looked at photos of themselves. Put simply, confronted with her own image, the anorexic's brain partly shuts down. The implications for our understanding of the illness are significant, argues Mondraty, a psychiatrist at the Peter Beumont Centre for Eating Disorders. Once it's full-blown, he says, anorexia "is not really about societal pressures to be thin or about the patient being vain. There is a neurological disturbance here that makes it very hard for patients to get better."

Cause and effect are tricky to separate in mental illness, and anorexia, which kills between 5% and 10% of its victims, is no exception. Doctors don't know what causes the condition, though for a long time they've had a fair idea about who's prone to developing it. To some extent it runs in families, though it may be that some parents pass on to their children, genetically or by example, tendencies toward perfectionism, hypersensitivity and perseverance — traits that, combined with low self-esteem, appear to be preconditions for anorexia. Typically striking in adolescence, the disorder is more common in affluent countries and, within those, in wealthier families. For every boy who gets it, nine girls do. There have been attempts to explain anorexia in the same way most doctors account for depression — as resulting from an imbalance of certain neurotransmitters in the brain — but these have tended to lead nowhere. Since anorexia was named in 1868 by English physician William Gull, says Mondraty, "we haven't made much progress on treatment."

Mondraty and Sachdev, professor of neuropsychiatry at the University of New South Wales, saw the chance to make a small breakthrough in exploring why it is that the anorexic sees on her frame fat that isn't there. It's a puzzling, highly specific delusion: anorexic women can recognize normal weight in others and identify other anorexics as too thin, but something goes awry when they look at themselves.

Learning more about what that something is was the goal of the researchers, who presented their findings at the 6th International Congress of Neuropsychiatry in Sydney last month. They took photos of the 20 female participants — 10 anorexics and 10 controls of normal weight — all of whom were dressed in vest and shorts. Though faces in the pictures were deliberately made blurry, the women were told whether they were about to see others or themselves while in the MRI scanner.

The scans showed that when the anorexics and the controls looked at pictures of others, the type and extent of their brain activity more or less matched. But it was a different story when the two groups studied their own image. The controls' brains again lit up in predictable regions, but activity in the anorexics' brains was much more limited. Specifically, the areas involved in visual perception and emotional processing stayed out of play. Because the anorexic patient can scarcely bear to look at herself, Sachdev theorizes, "I think what the brain is trying to do is inhibit the level of processing. It will then distort [the self-image] so what the patient sees is based on preconception rather than on what is really being looked at."

But is this phenomenon a cause — even the cause — of anorexia, or is it a consequence? Mondraty sees it like this: the personality traits and self-esteem problems mentioned earlier "are reasons that a girl might go on a diet," he says. Nearly all girls diet during their teenage years, but only about 1% of them develop anorexia. "What I'm guessing," Mondraty says, "is that when those with an underlying biological vulnerability lose a certain amount of weight, then something happens ... this [brain abnormality] clicks in. The significance of this is that it takes a bit of blame away from the family, away from the patient, and lets us — the people who treat anorexia — realize how hard it is for the young woman to get better. We're telling her to eat when everything she's experiencing is telling her she's already fat."

Just to show that this is a complex field, Mondraty's coauthor sees things slightly differently. "I think what we've observed," says Sachdev, "is a functional abnormality that probably follows, rather than precedes, the development of anorexia." The pair plan to conduct a follow-up study on the 10 anorexic patients in about two years to see whether brain activity has normalized in those who've recovered. If it has, that would suggest that what they've observed is a product of the disease — triggered, perhaps, by malnutrition — rather than a hardwired abnormality. Brain-imaging skeptics would argue that all Mondraty and Sachdev have observed is an extreme example of what happens in the brain when we focus on something that makes us feel anxious and inadequate, or when dread causes us to mistake the harmless for the fearsome — the garden hose for a snake.

Whatever they've identified, both see merit in thinking about ways to treat the phenomenon. "I don't want to overstate it," says Mondraty, "but I guess in the future it'll be interesting to see if we can develop drugs that will actually reverse this abnormality." For some of his patients Mondraty already prescribes an atypical antipsychotic, olanzapine, to help control the intensity of their self-image delusions and curb their tendency to obsess about food and shape.

One of those patients is Nicole, who in Mondraty's care has gained 15 kg this year and started working part time as a receptionist, though she's still underweight and still thinks she's fat. "We have a long, long way to go," says her mother, Kathy. Despite this latest study, the same could be said, sadly, for our understanding of anorexia.

• Daniel Williams
• Can anorexic women see themselves as they really are? A new study suggest their brains won't let them