It was when his employers decided to move headquarters to Merseyside that Stuart Carter, then in his late 50s, reckoned it was time to retire. Stuart's career as a personnel manager had taken him and his family around the country - London, Manchester, Sheffield, Somerset, Surrey - but this next move seemed a step too far. He and his wife, Stuart felt, should stay put, and hold on to the roots they had laid down in the south-east. The only problem was that by 1997 Stuart had begun to find it increasingly difficult to stand still on the spot.

Initially, Stuart ignored his faltering balance, and the odd way his left arm refused to swing as he walked. He took serious note only after a GP friend came round to dinner one night and quietly remarked, "I see you've got the big P."

At first Stuart did his own research, ticking off symptoms from what he describes as the "à la carte Parkinson's menu". Rather than the shaking symptoms of the disease, he had the freezing ones. A referral to the Institute of Neurology in Queen's Square, London, led to medication that alleviated the initial constrictions. At first it was a relief to know that what was happening had an explanation; that it wasn't "all in the mind".

After Stuart's retirement, however, his symptoms began to creep over the border from the physical into the psychological. As a brain disease, Parkinson's operates between the "secondary effects" on the self that any physical illness may have (cancer will change the way you feel) and the "primary effects" of a psychiatric illness that directly distorts perception and cognition.

Stuart found that, because he could no longer will his face or voice to form familiar expressions, he himself was being diminished. It was a drastic transformation for a man who used to love amateur dramatics. During the 1970s, he had played Jesus in the Chester mystery plays, and his wife, Glenys, had dubbed him "the first bald Christ".

Now, as Glenys looked on, Stuart found himself looking expressionlessly back at her. Even his voice seemed to be fading away. He was taking eight tablets of the Parkinson's drug Levodopa, along with as many as 21 other pills a day, but the periods of relief were getting shorter. It was as if he were a statue on a plinth, staring out through a face of stone - one that was fixed like a mask of Greek tragedy.

The idea that neurosurgery might restore something of Stuart's self took a while to emerge. But in recent years, a new technique has been developed to target a small, lens-shaped area deep inside the brain, called the subthalamic nucleus. Eighteen months ago, Stuart was approved for a process called deep brain stimulation, in which a hole is drilled into the top of the skull, and an electrode is inserted down into a spot that Professor Marwan Hariz, who performs the operation at the Institute of Neurology, describes as a roundabout for the neural traffic that mediates the symptoms of Parkinson's. A high-frequency current delivered into such traffic can divert it, helping to liberate the patient from his symptoms.

In Stuart's case, the surgery came with an additional feature. A BBC crew was filming procedures for a series called The Brain Hospital, and asked him to take part. This, he felt, would be an opportunity, once again, to project himself - to inform the public and offer hope to other sufferers. What he didn't expect was to find himself struggling for his life due to another, unrelated problem. After his first operation, Stuart developed a pulmonary embolism that sent him into intensive care for 15 days. Even as Stuart's life hung in the balance, Glenys told the BBC that they could keep filming, because she knew it would be what he wanted.

Stuart survived. But a second operation to install the stimulator in his chest, connecting it to the electrode in his brain, was delayed for another six months, by which time the series had aired, leaving viewers uncertain what had finally become of him.

What, in fact, became of Stuart is remarkable. Glenys describes it as a sort of return. The ultimate march of the disease remains inexorable, but he has been granted an extended lease on selfhood.

Taking less than half his previous dose of Levodopa, Stuart has regained essential forms of volition. His face and voice do what he tells them. He can stand or walk at will.

And because there were cameras there to watch as a wire was guided into his head, he had an opportunity, at the age of 66, to perform a crucial role in illustrating an extraordinary achievement in medical science and, now, to help provide hope to other sufferers that they may yet become themselves again.