Susan Greenfield spends a lot of time thinking about thinking. Now she's working on a drug to treat brain diseases that cripple millions. Alison Goddard reports.

What do you think I would know or do if the puzzle of human consciousness had been solved - if we understood exactly how we experience the colour green or the smell of coffee brewing or the pain of a grazed knee?" The query hangs in the rarefied air of London's Royal Institution, where the bright and breezy Susan Greenfield, leading researcher and media darling, is director.

"Would I be able to climb into your brain?" she asks. "Would I be able to share your consciousness, in which case you would be denied your innate individuality? What exactly could I do with the knowledge of how human consciousness happens?" The riddle of consciousness has troubled Greenfield since she was a teenager and her mother told her that neither of them would ever know what the other felt when they saw the colour red. But why is the puzzle so difficult to solve? "The big problem is asking the right questions," she says. "The ultimate question is how do physical events within the brain translate into subjective experience? How, for example, does changing the availability of the chemical serotonin translate into a subjective feeling of well-being?" How, in short, does the brain work?

Greenfield is as close as any of the many scientists investigating consciousness to finding at least part of the answer. As professor of synaptic pharmacology at Oxford, she is exploring connections between the brain diseases Alzheimer's and Parkinson's. "My view is that the two, although they exhibit different symptoms and are treated by different medication, share common factors. What we think happens, in both cases, is an aberrant form of brain cell development. For some reason, the brain cells of sufferers think they should be growing again. They bring into play mechanisms that are fine when a foetus is developing but are pernicious in old age."

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Last year she launched a company, Synaptica, with three other academics, David Vaux, Nick Rawlins and Martin Westwell. One potentially lucrative goal is to identify drugs to treat Alzheimer's and Parkinson's.

"We are trying to discover the critical chemical that is common to, and might be responsible for, the brain cells that are lost in Alzheimer's and Parkinson's," Greenfield says. "If we could discover it and characterise its action, then Martin could design a drug that would tackle both diseases, a medication that would arrest degeneration of the brain.

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"The market could be huge," she adds. "It is predicted that by 2050 in the United States alone, as many as 14 million people could have Alzheimer's. And it is not just patients who suffer. Arguably Alzheimer's is more stressful for relatives. The patient is unaware of what is happening, whereas for the carer it is devastating to watch the senility of someone you love."

A better understanding of the brain would also help Greenfield unravel consciousness. But there are many barriers to hurdle first. She is enthusiastic about the relatively new diagnostic tool of brain imaging, whereby a scanner is used to produce a picture showing which parts of the brain are used for different tasks. But she worries that brain imaging alone "cannot inspire a theory of how the brain works. All it reveals is that if you do this, bits of the brain light up - it doesn't tell you why or how or what that means."

In fact, perhaps surprisingly for a woman so enthusiastic about life in general, Greenfield is downbeat about many of the grand claims made by scientists at the end of the 20th century. Of the geneticists' hope that we will soon be able to pinpoint and modify genes for human behaviour, genes for shyness, for instance, or intelligence - she is dismissive. "I think people may be disappointed by the mapping of the human genome," she says. "Even if you know what a gene is for, you don't know how that relates to the function of the whole brain. You have 1 million genes and 100,000,000 million brain connections. So there might be an over-expectation about what molecular biology can deliver. It is not conceptually viable to find the gene to make people fall in love with you."

Nor will the artificial intelligence community succeed in their goal of building a conscious robot, she says. "The idea is ridiculous. Consciousness entails an interaction between the brain and the body, trafficking a myriad of chemicals between the two. To reproduce that, you would have to build a body with a whole range of chemicals, and the three-dimensionality of the brain would have to be preserved to the very last connection."

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But if scientists do not have the answers, nor, according to Greenfield, do philosophers, perhaps the first people to define the problem of consciousness. "I can't see how philosophy on its own is going to explain how the physical brain is translated into subjective experience," she says. "I think their role - and I shall probably offend them hugely by saying this - is to provide checks and balances on any theories a scientist might come up with."

Greenfield's career has been primarily based at Oxford, with spells in France and the United States. Now 48, she married Oxford chemist and millionaire Peter Atkins in 1991. The first woman to give the Christmas lectures at the Royal Institution and author of several popular books on how the brain works, she has a new six-part BBC television series coming out in April 2000, provisionally called The Secret Self. "I can unleash on an unsuspecting nation my ideas on how the mind works," she enthuses.

I ask her what it is like to be a female scientist. "When I was younger, I did not see it as a problem. Paradoxically, it was only as I got more senior that I realised there was a problem. Previously, when I was patronised, I thought that was because I was junior," she says. "If you feel upset by something, my advice is go and talk to another woman. It is a way of channelling anger. Otherwise you get labelled prickly and humourless."

Although she is not a mother - "I had a small brother and was disabused early on about what babies are like" - Greenfield is angry at how women scientists still have to choose between children and a career.

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She also wants to see female scientists paid more. "What saddens me is the women themselves: they feel like impostors. We need a change of mindset to help women get a sense of their own worth and to help them handle confrontation."