SUSAN GREENFIELD needs little prompting to recall when she first held a human brain in her hand. It was more than 30 years ago, when she was in her late 20s. Back then, she says, she hadn't realised you could study the brain in any meaningful way; it was just there, "a constant presence behind your eyes and between your ears, encased and inaccessible in your skull". How, she pondered, would cutting up a dead brain reveal how it works?

Three decades on she knows a lot more but the brain is still as mysterious as ever, the one part of our bodies that defies explanation. Brains, says Greenfield, as if she were discussing cuts of meat with her local butcher, have the consistency of eggs. Poached? Scrambled? Fried? More like soft-boiled, she says. "It has a form. It doesn't just fall apart like an egg yolk."

She wore surgical gloves because the brain she was about to touch was bathed in formalin. "I don't know if you've ever smelt formalin?" she asks. I confess I have not. On the plus side, formalin makes brains firm and cuttable; the downside is that it's highly toxic and smells like a camel's breath. "It's really not very nice," she adds unnecessarily. What if she hadn't been wearing gloves? "Just say that the brain tissue, though firmed up by the formalin, nonetheless gave way under my inexpert heavy hand, and lodged under a fingernail."

It is not a thought I like to consider too deeply, it being not long since breakfast. Greenfield, however, continues as if she could happily dissect brains while simultaneously spreading marmalade on toast. The human brain, she explains, is constantly evolving and adapting to accommodate changing circumstances and environments. It may feel like something you want to dunk soldiers into, but it is a complex organism, capable of performing feats which have allowed mankind to create a highly sophisticated world out of unlikely matter. It is also, as Greenfield knows as well as anyone, the locus of diseases such as Parkinson's and Alzheimer's, which remain elusively incurable.

Ever since she first handled a brain, Greenfield has been obsessed by Alzheimer's. It is, she says, "the core thing I do". One of Tony Blair's people's peers, appointed to put intellectual sparkle into the House of Lords, she is about as eminent as one can be without being a Nobel laureate. Her CV is as packed as a supermodel's handbag: she is the professor of synaptic pharmacology at Lincoln College, Oxford, and the Chancellor of Heriot-Watt University. Moreover she has so many honorary degrees she'll soon run out of universities to give her new ones. She has presented television programmes, founded research companies and been awarded a CBE for popularising science. The publication of her latest book, ID: The Quest For Identity In The 21st Century, coincides with the re-opening of the Royal Institution, of which she is the first female director in its 209-year history. Among the previous incumbents were Sir Humphry Davy, inventor of the Davy safety lamp, and Michael Faraday, the electricity pioneer, at whose desk Greenfield works while a hard-hatted army rushes to complete the multi-million pound renovation of the imposing pile in London's Albemarle Street.

Lest that leaves the impression of a white-coated boffin who arrives at the office by Tardis, Greenfield, who was once judged to be the 14th most influential woman in the world (albeit, as she is always keen to point out, five places behind Dolly Parton), is dressed like a teenager for clubbing: short skirt, goth boots, blinding blouse and flowing locks. She is petite and wired to the National Grid, taking calls, instructing her secretary whom she hired after they met while she was "thinker-in-residence" in Adelaide, and talking 19 to the dozen, a lava flow of words reflecting her impatience to cram as much into her day as possible.

Famously, despite her numerous achievements, Greenfield has yet to be invited to become a member of the Royal Society, the oldest scientific society in the world and a self-electing body which takes a rather snooty, male chauvinist attitude towards the likes of Greenfield, who some suggest has committed the unpardonable sin of trying to interest the public in science. Apropos of which, she says: "You know the Kissinger quote: University politics are so spiteful because the stakes are so small.'" What she finds particularly galling is the media's habit of using unattributed quotes, allowing those who criticise her to hide behind the cloak of anonymity.

"That I find troublesome," she concedes. "That's the kind of environment that would prevent scientists engaging with the press. We live in such a risk-averse society that people are frightened to have a theory in case it's wrong. What is wrong with being wrong? Where's the dishonour in having a theory that turns out not to be right?

"I find it sad when people only do things they know are proven and are frightened of any controversy. I'm particularly sad in my own case when it's anonymous. If I ruled you guys, the press, I'd have a rule: it would be any quote you use has to be attributable. So that if these people want to say things, that's fine, they can say what they like, but they have to say who they are so they're accountable for what they say."

It is said with feeling and, doubtless, justification. Another danger of the press's coverage of science, she notes, is that it can raise hopes without justification or before tests on medical advancements have been properly concluded.

"I don't care about how you portray me - well, obviously I do - but the more important thing and what I really do get upset by, is if you raise people's hopes and dash them. It is criminal, really criminal. These poor people have enough to worry about: they've got suffering, they're desperate, they really want to believe this news. So even the slightest hint, unless it's really well qualified, will raise hopes and expectations and excitement. Because when people are in extremis, hope is the only thing they have, and they will cling to anything. It's immoral, that's the word. It's immoral to do that just to sell a few papers."

No-one is more aware than Greenfield, however, of the urgent need to find a panacea for Alzheimer's and its sister disease, Parkinson's. The two, she says, contrary to the beliefs of some of her colleagues, are related. Very frequently, people have Alzheimer's and Parkinson's together. Another intriguing factor is that if brain cells are damaged it is not necessarily the case that you will get either disease. She says she is "constantly frustrated by the fact that current therapeutic strategies to halt this terrible dismantling of the mind" do not tackle the root problem: what is it that causes the underlying key brain malfunction.

I tell Greenfield that my mother, like a lot of sufferers from Parkinson's, was given L-Dopa, a powerful drug which for a short time restored her to normality. She could not see why she was not prescribed it habitually. "I've heard people with Parkinson's say they would rather not move at all than have the excessive tremor that comes on because people think they're drunk and so on," says Greenfield. Such drugs, she adds, can only deal with a certain part of the brain. What they can't do is tackle the underlying cause. She compares it to taking Contac 400; it will stop your nose running but it won't cure the cold. "You can treat symptoms but that's not necessarily tackling the disease. So you get temporary alleviation, which is fine, but let's not confuse that with getting to the heart of the problem."

What is beyond doubt is that the prevalence of Alzheimer's and Parkinson's is increasing and, with people living longer, the situation is bound to get worse before it gets better. At the moment, 700,000 people in the UK have Alzheimer's, while 120,000 have Parkinson's. Estimates suggest those figures are likely to double by 2050. The financial cost is potentially ruinous. The human cost is incalculable. For, unlike, say, cancer and heart disease, Alzheimer's and Parkinson's reduce their sufferers to shells.

"Cancer and heart disease are serious illnesses," says Greenfield, "but you are still the person you were. Whereas with Alzheimer's quite often what you find is the relatives are going through a period of bereavement while the person is still alive, with a sense of loss just as though the person had been killed in a traffic accident. But whereas society will accord people time and understanding if they've got a relative who's died in a road accident they don't realise, because the person's still alive, that they need to have the same period of grieving."

In ID: The Quest For Identity In The 21st Century, Greenfield writes apocalyptically: "The slow, daily erosion of the mind is a relentless process that we can't control, where we have seemingly done nothing to deserve such a devastating endgame, and where there is no restorative therapy. But it is not just an increasing number of patients' lives that will be blighted. Ask yourself this question: how many people love you, or let's just say, care about you a bit? Let's assume 10 ... So say that for every one person suffering from degenerative disorders, there are 10 that care about that individual: if by the middle of this century numbers of those suffering reach in the UK the expected two million, we could be looking at some 20 million lives devastated: roughly one in three of the whole population of the entire UK!"

If that sounds grim and unscientifically alarmist, Greenfield is no more reassuring on other fronts. The thrust of her new book, for example, is a rumination on what the effect is on our brains of the way we live now in this era of video games, mobile phones, constant internet access and inescapable TV. How our brains cope with such a blitzkrieg is anyone's guess but Greenfield's is that it is not good. In contrast to previous generations, brought up to socialise with their peers and behave responsibly in public and share experiences with their family, today's children, who spend much of their time alone in their rooms playing games by themselves, are less likely to develop as well-adjusted human beings. The consequences for society at large are potentially frightening. For instance, studies seem to indicate that there has been a marked decline in the verbal communication skills of children. Also, says Greenfield, the rise in Attention Deficit Hyperactivity Disorder (ADHD) and the subsequent threefold increase in prescriptions of the drug Ritalin over the past 10 years indicate that what was once a relatively rare condition is now becoming quite common.

In the absence of hard evidence, Greenfield is inclined to identify computer screens and TVs as the source of the problem. Park a two-year-old unsupervised in front of a TV, she says, as if it were "a kind of electronic supervisor", and he or she will soon realise that they can make things happen very quickly by pressing the remote control. Thus their environment is conditioned by action-reaction, action-reaction.

"You then go to school and you're required to sit still for half an hour. And you're used to action-reaction, action-reaction. What are you going to do? You're not going to sit still for half an hour. And then someone's going to say you've got ADHD and they're going to give you Ritalin.

"Obviously you need to give drugs when someone's in pain whether it's mental or physical, but any drug will have a very powerful effect on the brain. There were 55,000 prescriptions in the UK this year for Ritalin; I cannot see how that can be without side effects or effects of some sort."

Much the same, she adds, might be said of cannabis, moves to decriminalise which she denounces as a sop to the youth vote. "I hate that word experimenting' with drugs. It dignifies it. It's ludicrous." Nor is she any more enamoured with David Cameron's "hug a hoodie" idea. "It's so facile." She'd rather politicians invested money and resources into exploring why people take drugs, drink themselves silly and behave as they do. "Is it because life is so shitty that you want to escape it?" But Susan Greenfield knows that politicians suffer from ADHD as much as kids and that long-term policies have as much appeal to ministers clinging desperately to power as Christmas has to turkeys. And that, alas, is a scientific fact.

Susan Greenfield's new book ID: The Quest For Identity In The 21st Century is published this week by Sceptre, £16.99