HEADING south on the M25, Anthony Robertson starts to guffaw. His mouth stretches into a cavernous grin, his head lolls back, his arms beat up and down and a primal, animal-like 'uoerrrrr . . . uoerrrr . . . uoerrrrgg' jerks spasmodically from his throat. To the uninitiated the sound is both disturbing and compelling. His personal assistant, David Hodgson, attempts to share the joke, glancing sideways and murmuring, 'Yesterday? This morning?', alluding to some prior shared incident. While I wonder what it is that Anthony - a 20-year-old with cerebral palsy whose voice, arms and legs have never functioned - has to be so amused about, I listen again to that laugh. And then I realise why it is so utterly compelling - it is the sound of a man with no voice laughing.

In fact, Anthony does have a voice, but at that moment it is sitting silently at the back of the van, wrapped in a plastic cover. It comes in the form of the Liberator speech synthesiser - a pounds 6,350 computer equipped with an electronic voice box - which is arguably the fastest, most sophisticated communication aid on the market today. As the UK's most proficient user of this nifty piece of equipment, Anthony is employed by Liberator's distributors as an ambassador for the company. On a salary of pounds 16,000 and with a shiny new Nissan Vanette at his disposal, he travels the length and breadth of England, setting up meetings with people who cannot speak - the users and potential users of communication aids.

Today he is on his way to Kent, to demonstrate his kit to the parents of a boy with cerebral palsy and to take part in a users' meeting at Nash House, an educational college for teenagers with disabilities near Bromley. 'Talk with me - it's the body that's unwilling, not the brain,' proclaims a sticker on the windscreen of his van.

Anthony was 11 months old before his mother, Doris, discovered he had cerebral palsy. 'His hands were always clenched and he used to cry 24 hours a day but, that apart, he was such a normal boy,' she says.

'OK, he wasn't crawling or sitting up but I suppose I tried to convince myself that he was a bit slow. When the doctor told me he had cerebral palsy, I was so confused - I didn't really understand what caused it or what it meant.' The cause of the condition has yet to be discovered but it is known that it involves damage to the part of the brain that controls muscular movement, and that for the 1,500 babies born with cerebral palsy in Britain every year (one in 400 births) there is no cure. Indeed, finding a cure is complicated by the fact that no two people are affected alike. The degree to which muscular functioning is impaired varies from extremely severe, as in Anthony's case, to barely noticeable. Because people with cerebral palsy may be unable to control their facial expressions, it is often assumed they are mentally impaired as well. But mental damage is rare and many, like Anthony, have higher than average intelligence. Unlike muscle- wasting diseases, the condition is not progressive, which means that cerebral palsy sufferers do not deteriorate and can live long and otherwise healthy lives.

But one thing that people like Anthony could not do was talk. Until he acquired a Liberator two and a half years ago, Anthony's sole means of communication was a chart with symbols called a Bliss Board. By directing a beam of light from a light pointer attached to his head (the one part of his body he can control), he could illuminate symbols - for food, drink or the lavatory for instance - and his carer could interpret his needs.

The majority of the speech synthesisers available - such as the one owned by Professor Stephen Hawking - use a typewriter keyboard and require the user to know how to spell, a skill that most people with cerebral palsy never acquire. The inspiration for a machine accessible to non-literates came from the American researcher Bruce Baker who was studying Egyptian hieroglyphics. He became fascinated by how a non-literate culture used a small number of symbols to attain such diversity of meaning and he developed a symbol- based software package that did the same. The image for 'sun', for example, could be used to represent the colour yellow, the concept of roundness or heat, depending on its use in combination with other keys. The user activates each key either with his hands or, as in Anthony's case, with the infra-red beam of a device attached to his head called a headpointer.

It was not until the mid-Eighties that Liberators were imported into the UK, making Anthony's the first generation of British people with impaired speech to have the opportunity to communicate audibly. There are more than 3,000 users of speech synthesisers in the UK and approximately 70 per cent of them use Liberators.

Anthony wheels into the meeting at Nash House, his speech synthesiser mounted on his wheelchair, the headpointer protruding from his forehead like the horn of a unicorn. 'It . . . is . . . good . . . to . . . see you . . . . It is good to see you,' says Anthony to another user, his voice box sequentially pronouncing each word as it is activated with a twang by the infra-red beam and then repeating it all as one uninterrupted sentence.

'My name is Louise,' says one girl. 'I'm 16.'

'Oh, to be 16 again,' replies Anthony.

The banter is flirtatious and, despite the drawn-out delivery, spontaneous. The conversation moves on to standard adolescent subjects - dating and pubs - before descending into anarchy with everyone using their voice boxes at once. There they are, a dozen young men and women without natural voices, but conversing animatedly nevertheless in a variety of pre- programmed American accents.

Liberator users have a choice of nine quirkily named voices. Anthony uses Perfect Paul with its measured tone, but he can switch to gruff Huge Harry, effete Frail Frank or the officious Doctor Dennis. For the women, there's Whispering Wendy, Uppity Ursula, Beautiful Betty and Variable Val, and for children, Kit the kid. Within each voice, the sophisticated user can modulate pitch, assertiveness and even breathlessness. Unlike Anthony, most of the users are novices though, communicating in short bursts with strange intonations and less than perfect grammar.

'The way Anthony uses his talker is extremely sophisticated,' says Judy Hensman-King, a speech therapist and, in a more personal capacity, Anthony's partner. 'For example, if something isn't in his vocabulary, he'll put the words together phonetically. When he wanted to say 'specialise', he used the word 'special' and added 'eyes'. The company flies him to the US to give speeches. He's their worldwide prodigy, a shining light. Talking to Anthony is like talking to anyone else, just a little slower.'

But is this last assertion really true? It is certainly the case that Anthony holds his own in conversation, often making his points with witty and astute use of the one-liner. Asked, for example, whether there is anyone he cannot communicate with, his reply is sharp and to the point: 'Only those who will not listen.' Perhaps that ought to be enough, but in responding to questions on his personal life, his answers seem stoical, verging on the superficial, and I can't help feeling frustrated by what is either the guarded response of a vigilant salesman or a real limitation in the emotional vocabulary of his synthesiser.

How did you feel back there in the car when we were talking and you couldn't join in? I ask. 'I guess I take that as life - I don't really think about it,' he says. What was it like growing up without a voice? 'Really, I didn't know any different. OK, I got mad a bit that I was unable to do things like my brother but I took that as being part of life.'

But what did it feel like? 'Just different. A voice means that I have independent communication. I can call across a crowded room. I don't need someone to talk for me. I can speak for myself.'

'What are you thinking?' he asks, observing my furrowed brow. I take a breath. Somehow it doesn't seem fair to pierce the armour, to ask the questions that I would of an able-bodied interviewee. There is a pressure, in the interests of political correctness, to gloss over differences, to pretend that his laugh sounds like any other, to take it on trust that he can communicate like the rest of us. Yet in Anthony's eyes - his only means of expression until the age of five - is a look that demands nothing less than total honesty.

Well, I say, you can speak, but I can't help wondering whether the Liberator really allows you to express yourself? There is a silence and for the first time he pauses before replying. 'There is more to that than 'no' or 'yes',' he says. 'Although it is true that I can't say exactly what I'm feeling when you ask me complex questions about my childhood, it's so much better than what I had. Before I was powerless, institutionalised, dependent. People would ignore me because I couldn't speak. That doesn't happen any more. But let's be real - nothing could replace having a normal voice.'

One of Anthony's gripes is that state funding for speech synthesisers can be difficult, and in some cases impossible, to obtain. Recently, he and two other users, Peter Zein and Trevor Down, formed a sub-committee to lobby Parliament for clarification on funding. Speech synthesisers, they argue, are as essential to them as glasses are to people with defective eyesight.

Peter suggested they chain their wheelchairs to the House of Commons in protest but, after weeks of drafting and re-drafting, a letter to Timothy Yeo MP, then Parliamentary Under-Secretary in the Department of Health, was dispatched instead. The reply, dated 25 May 1993, states that 'there is no difference in the legislation applying to speech synthesisers as compared with the provision of wheelchairs . . .' and that funding is provided by the NHS or education authorities. Anthony insists that this is not the case. .

One hazard of speech synthesisers is that the public, curious about this novel talking machine, sometimes barge in on Anthony's private conversations. Later, in a pub, while Anthony and Trevor are telling me that neither of them are disabled in their dreams, a bleached-blonde woman in high heels clicks over to the table and bends over Trevor's shoulder. 'Hello dahlings]' she says. And then slowly enunciating each syllable, she continues. 'What . . . game . . . is . . . this? What's . . . your . . . n-a-m-e?'

'My name is Trevor,' he replies, continuing with his supper, (and unwittingly demonstrating how, unlike natural-voice speakers, he can talk and eat at the same time.).

'And . . . my . . . name . . . is . . . Alex . . . and . . . I'm . . . the . . . landlady . . . here,' she says, before breaking into a soliloquy on her 'long, arduous day'.

'Hard life,' says Anthony, his sarcasm swamped by her parting gush.

'Gosh, she was condescending,' mutters Judy furiously. 'She never once looked at Trevor, just over his shoulder at his machine. And the way she slowed her voice as if he was retarded. People see someone who can't speak and they think they're stupid. Meanwhile, he's probably brighter than she is. Sometimes when I'm out with Anthony, people say things like, 'Oh, isn't he enjoying himself]' and 'I bet it's great to be out]' I just want to turn round and say, 'Anthony is never in, you know'.'

Anthony's arms windmill about as he aims infra-red beams at his Liberator. 'Nine times out of 10, I am patient and tell them about the Liberator,' he says. 'But sometimes, after a hard day's work, I prefer to leave my Liberator at home. I am just a person. I am not on show.'

(Photographs omitted)