It's a chilly day but the excitement at the Patrick Wild Centre for research into autism and intellectual disabilities is generating plenty of heat.

In its cramped laboratories overlooking Edinburgh University's George Square Gardens, researchers are wearing clean white coats, the best table in the common room lies empty, the VIP sandwiches are chilling in the fridge. Dame Stephanie Shirley, IT entrepreneur-turned-philanthropist, is touring the facility. It is the first time she has visited since donating £1 million for a state-of-the-art imaging suite. If ever there was a time for best behaviour, this is it.

Since 2001, Dame Stephanie, 79, has donated £50m to autism research. Unlike many philanthropists, who prefer flashy, one-off projects, she is happy to invest in infrastructure, in this case a roomful of microscopes and computers where researchers hope to advance their work on the molecular and cellular basis of autism.

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The Patrick Wild Centre focuses on Fragile X Syndrome, its most common genetic form. By looking at how neurons communicate in the brain, what goes wrong, where genes go awry in development, they hope to gain insight into this hugely complex condition. For parents of autistic children, this is hugely encouraging.

For Dame Stephanie, whose son Giles was autistic, it is her passion. She has also founded Prior's Court, an independent school for students with autism in Berkshire, one of the most advanced in the world, the website autismconnect and Oxford University's Brain Bank, where scientists are looking at the cerebral matter of 17 autistic people, including that of Giles.

When Dame Stephanie's only son was born in 1963, there was no treatment and little understanding of autism and Giles soon changed from adorable baby to uncontrollable toddler. Aged two and a half, he lost what little speech he had. After 10 months of tests he was diagnosed with severe autism, with the recommendation that he went into a home.

"It was the habit then to institutionalise children," Dame Stephanie recalls. "He was very wild, but at that age I could still tuck him under my arm and cope.

"We decided not to have any more children because I could not have got through a pregnancy with Giles at home. Although I regret not having had any more and not having any grandchildren, it was the right decision. We decided to concentrate on the child we had and keep him in our home. We managed to do this, with difficulty, for 13 years."

And while Dame Stephanie and her husband, Derek, were living with an uncommunicative, wakeful child, they were also in the early phases of starting their IT company, FI Group. Back then, she thought her life's work was to create flexible jobs for women in the computer industry and, until the Sex Discrimination Act made it illegal in 1976, all her staff were female.

Although establishing a business while caring for an autistic child sounds like a recipe for a meltdown, she says it was her salvation. "If it hadn't been for the business, I'd have gone stark staring mad. There I dealt with problems I could solve. One of the sad things about autism is that the child is totally oblivious to everything you do."

Working in shifts, the couple cared for Giles. Then, at puberty, when he became violent and developed asthma, the strain proved unbearable and Dame Stephanie eventually melted under the pressure. She ended up in hospital for a month while Giles, aged 12, was admitted to what she calls "an old-style hospital for the mentally subnormal".

For the first few years, he was treated well. But, from the age of 16, by which time he was also epileptic, his parents were unhappy with his regime. "He was locked up and started to lose some of his human rights," she has said, declining to elaborate. When he was 23, the couple brought him home and Dame Stephanie started a care home in her hometown of Henley-on-Thames where Giles became the first resident. Eventually he became a little more independent, a little less violent. He could go to the shops, walk in the woods, brush his hair. In 1998, aged 35, he died during an epileptic fit.

By this time his mother had already transferred 26% of the company, now called Xansa, to her staff, creating more than 70 millionaires, and retired in 1993 to concentrate on philanthropy.

Dame Stephanie is a rationalist. She approaches the disbursement of her considerable fortune – estimated at £150m at one point – with her head rather than her heart: "People say sentimental things like 'it's all in Giles's memory'. It isn't. But it is very much to obviate some of the problems our family had."

Her lack of interest in legacy projects can be partly explained by her extraordinary background. She was born into a Jewish family in Dortmund in 1933. When her father, a judge, was fired by the Third Reich, the family moved around Europe. At the age of five she was sent alone by train to the UK, part of the kindertransport rescue mission. Arriving at London's Liverpool Street station, she was classified as a "friendly enemy alien".

She had no money, no passport (Hitler had stripped Jews of their German citizenship) and not a useable word of English. A childless Quaker family adopted her and brought her up as their own.

Coming to Britain with little more than a yellow star and a teddy bear (both now in museums), being welcomed into a family and then a whole culture, has, she says, informed her whole life. "It made me realise that I had to make the life that was saved worth saving. Today, 70 years later, I still feel I've got to do something worthwhile with each day and not just fritter it away shopping. Although I can get quite extravagant over certain things, basically I'm a very worthy person.

"I feel I need to give something back and if I can work in the public service, it's just a tiny repayment for the welcome I was given. I can get very sentimental about that."

The redistribution urge started early in her career. In the early 1970s Dame Stephanie persuaded the board of the company to give 1% of its pre-tax profits to charity. She had decided early on that the workforce should own the company, along the lines of the John Lewis model. There were also donations "of much smaller magnitude than I give now, but which cost me much more. If we made that gift, we wouldn't be able to have a new car. It hurt".

As her knowledge and experience has grown, Dame Stephanie has become bolder and more discerning. "The projects I supported in the early days – and they were perfectly good projects – were by chance. Somebody asked me [and I said], 'That sounds very good, have a cheque.' Today I'm very scathing about just writing a cheque.

"Many charities come to me with the most dreadful needs, feeding an area of Africa or dealing with animal cruelty. Really needy projects. But that's not what a philanthropist responds to. That's the Government's job. Philanthropists fund things the Government won't touch. Governments follow what we do, and when we've shown that something is possible, then it will cascade down."

Today, her criteria is different. To attract her attention the work must be "pioneering: I never just do more of the same, no matter how worthy". It must also be "strategic: so if it is successful – and pioneering projects often fail – it makes a real difference".

"The work here at Edinburgh is obviously strategic. They have been able to engender autistic-like behaviour in mice. The mouse becomes a bit anti-social, it cowers in the corner, it loses its little squeak. That's quite something to be able to do that but, more excitingly, they're able to reverse that and get the mouse back to its normal behaviour. Parents are so excited. It implies that, within the lifetime of their children, we're going to be able to do something like that."

As she tours the Patrick Wild labs, her questions are entrepreneurial rather than scientific. At the first mention of clone receptors she turns the conversation in a different direction. "What makes a good day for you?" she asks a young researcher. "Who will operate the new imaging equipment?" she asks a skilled technician. "What will you be doing in three years' time? What difference will the new equipment make?"

All the while she is assessing the long-term viability, the atmosphere, the personnel. "Sometimes I will support a little start-up, but generally I want a bit of stability. The work should be sustainable. It can't be just dependent on Peter [Kind, the centre's co-director]. If he fell under the proverbial bus tomorrow, would this work continue?"

Nine months after her visit, Kind is in perfect health and the centre's research continues apace. Dame Stephanie is back at home in Henley-on-Thames where she and Derek live "very modestly". Callers are astonished, she says, when she answers her own phone.

So does she still have more to do? She laughs. "I keep saying this is my swan song, then something else crops up. Money that's not working has a sort of obscenity to it. It's my job to keep it working." n