I suffer from clinical depression and have done on and off since my teens. The current episode has lasted more than five years and it looks quite likely that I shall always have a predisposition for the big gloom. I am one of the 20% of the UK population who live with this invisible illness and whose core symptoms include anhedonia.

This is a little-studied area of depression. It was first observed by the Victorians, but has prompted little investigation since. Someone suffering from it has little or no ability to feel pleasure from normally pleasurable experiences. Basically, it sucks the joy out of most things that make life worth living - food, sex, relationships and achievements. They no longer make sense as something which would prompt an emotional response. It's emotional dyslexia, if you like. You know what the occasion is; you just can't rise to it.

My own depression came with a variety of symptoms, few of which could be measured effectively and most of which have shifted and changed as the condition has become chronic. It's an illness which other people can't see, and unless they've experienced it first hand they sometimes view a sufferer as not helping themselves, as becoming a "victim of their own misery" and basically not even trying. Plenty of well-meaning people have said this to me in so many words and I can remember, to my shame, that I once said it myself to a depressed friend.

At the time I couldn't understand why she wouldn't just drop it; if she couldn't get a life at least she could try a hobby to help her get a grip. Now I know - when I am at my lowest, I couldn't "drop" my depression if the pope asked nicely, and I could no sooner get a grip than fly.

Depression can be triggered by specific life events, a physical illness, or a chemical imbalance in the brain. It seems likely that my present onset was prompted by a series of common disasters mugging me. My depression can be devastating. It can take every ounce of energy I have just to sit still; it requires every bit of spirit I possess to do anything. Anhedonia leaves me impassive and joyless. My responses become shallow and mechanical and I'm left feeling completely isolated and adrift from the world.

Even now that I'm better able to manage my symptoms, I still have to listen to other people's reactions to my achievements before my autopilot kicks in and I can at least appear more convincingly pleased.

There are people who just don't believe in depression. It's a subjective issue, of course, and with no visual aids and only the word of sufferers, it's hardly surprising.

There is hope, however. Dr Tonmay Sharma, a consultant psychiatrist at the Institute of Psychiatry in London, heads a team presently investigating the organic nature of depressive illness by conducting a study into anhedonia and depression through brain imaging. Functional magnetic resonance imaging - fMRI - has given researchers the opportunity to visualise the effects of depression on the brain.

His findings suggest that a tangible change occurs in the hippo- campus (an area of the brain which deals with emotion): it's often smaller in people with depression and anhedonia. Lesions and changes in brain metabolism have also been noted.

Dr Sharma's study has shown that the network which modulates mood in the brain of an anhedonia sufferer shows no activity when presented with pleasing or positive images, whereas in the brain of a person with no anhedonic symptoms such stimulation causes the network to light up.

"This is a crucial juncture in research into depression. The emergence of new technology is allowing us to map the brain," says Sharma. "We can now work together with patients to learn more about the condition and find appropriate psychological or pharmacological treatments."

The team is hoping that being able to show physical evidence in the brain will also encourage depression and anhedonia sufferers to obtain treatment earlier. He points out that if someone breaks an arm they will generally go straight to casualty, whereas someone with anhedonia might well delay asking for help in the hope that their symptoms will go away on their own.

Anhedonia responds well to treatment in most patients - whether it's drugs or therapy - and it's thought a delay of this sort can cause further dynamic changes in the brain, making the condition more difficult to treat. Sharma and his team also hope to investigate how treating anhedonia affects the make-up of the brain, again using fMRI. Sharma sees this study as an important part of the campaign against stigma being waged by the Institute of Psychiatry, as well as an exciting breakthrough in understanding the illness.

Enjoyment is a basic requirement for a full and happy life. We know that happy people live longer, and they generally cope better with illness and pressure along the way. The loss of this function can be truly crushing. Rats placed under unpredictable mild stress exhibit signs of anhedonia by a reduction in their response to reward.

Laughter and pleasurable experiences in humans are known to decrease levels of the stress hormone cortisol and help fight off respiratory complaints by increasing the level of immunoglobulin in our system. People find their larger pleasures in myriad ways but anhedonics lose a sense of joy in even the most basic of everyday comforts. A really excellent bacon sandwich, a glass of wine or a loving hug no longer provide sufficient inducement or reward to keep them fully functioning.

Learning to live with anhedonia is a strange and tedious business. I hadn't imagined for a moment that I might have to relearn one of the most basic of human experiences and it's a great deal harder than you might imagine. I've had to learn actively to take pleasure in the small things of life: clean cotton sheets; Ella Fitzgerald beginning to see the light with The Ink Spots; or chatting to the cat.

I'm moving on to achievements next and will slowly work my way up to blowing my own trumpet. Or polishing it at the very least. I'm learning to look for enjoyment in my day. I practise and reward myself by working towards small goals with the incentives that work for me - I do the Hoovering and win Desert Island Discs, a cup of tea and a fig roll, for instance.

Next week is national depression week. I hope it generates more talk and more research. The more study into the symptoms of depression the better the likelihood of effective treatments being found. The better the public awareness and understanding of this condition, the more likely it is that depression will finally be seen as an organic illness equal in validity to diabetes or epilepsy.

And people might finally stop trying to get us to join a club and just enjoy life.

• Contacts: Depressives Anonymous, 36 Chestnut Avenue, Beverley, Humberside HU17 9QU, telephone: 01482 860619. Run as a source of support to sufferers, complimentary to professional care. MIND (Granta House, 15/19 Broadway, London E15, telephone: 0181 519 2122) publish a wide range of informative literature about all aspects of mental health.

Further reading: Pitt B and Calman M Down with Gloom! Or How to defeat Depression (1994) Gaskell £3.50 (incl p+p) available from: Defeat Depression Campaign (publications), 17 Belgrave Square, London, SW1X 8PG.