During the worst years of the second World War, a Jewish couple in Poland sought refuge from the Nazis on a rural farmstead. The farmer consented, but his protection came with conditions. The couple would live under the floorboards of his house, a space so confined that they could barely stand upright.

In exchange for food and sanctuary, one of the refugees, who was a weaver, would knit clothes for the farmer. There was one more condition: under no circumstances, the farmer insisted, were the couple to have sexual contact.

According to the memoirs of the Austrian-born psychoanalyst and concentration camp survivor Bruno Bettelheim, Recollections and Reflections, the couple eventually violated the agreement: they conceived a child named Anna. To protect her, they kept her existence a secret. Anna's infant cries were stifled by her parents' hands, and for two years the child grew in perfect silence.

When the child finally emerged from under the floorboards, in 1945, her subsequent behaviour alternated between bouts of violence and long periods of withdrawal. Anna was the perfect example of the feral child.

This story seems to contain as much dark fantasy as psychological truth; Bettelheim maintained an equal interest in autism (though his theories have long since been discredited) and the importance of fairytales. But when the composer Trevor Knight read the story of Anna 10 years ago, the image of the feral child took hold of his imagination, invaded his dreams, and it never let go.

He would have to go back to his own childhood to understand why, he says. "That feeling of being alone, of being able to regress into a kind of womb-like state, of words not coming out as you were thinking them . . . I suppose I was able to identify with these conditions of isolation and communication. Most people would say there's a sort of a romantic thing to the feral child being wild, being outside of society."

Knight does not immediately strike you as a wild man. A tidy figure with closely cropped white hair and small, neat glasses, he is a cautious speaker, content in the silence or furious sound of music, but not necessarily comfortable with words. As a musician, Knight's interests have moved from jazz-fusion to avant-garde pop, but he has also steadily eked out a position as a prominent composer for the stage. "In the past few years," he tells me, "I've been more motivated by visual art and performance art than I have by music. A lot of music that I've heard hasn't fired me."

Inspired by the story of Anna and several more like her - the Indian sisters Amala and Kamala, raised by wolves in the 1920s; Kaspar Hauser who emerged in Nuremberg in 1828 after spending 12 years in a shed and was the subject of Werner Herzog's film The Enigma of Kaspar Hauser - Knight is currently developing a performance piece called slat. In development with the visual artist Alice Maher since 2002, slat has evolved into a collaboration between musicians, designers and a Japanese Butoh dancer, all seeking to portray a disturbing world without language, without release, lurking below the floorboards.

Throughout the long and continuing development of slat, one thing has remained constant: the performance space. "The main idea I had was for a structure with people walking above and this activity below," says Knight.

"About three or four years ago I proposed the idea to [ experimental Dublin music collective] the Crash Ensemble. One note I had was that the whole ensemble would be under floorboards and the audience would walk on top of them. They would never be seen. It was an analogy for communication, with artists being below. The notion was also that the structure divided the conscious and the unconscious: below is what you want to remain hidden, like the artistic endeavour." He also likened the structure to a cow shed: "the place below it is where all the shit drops."

That seemed like a frustrated view of the arts. "That would lurk somewhere in it," he admits. "A frustration that things always had to be the exact same way."

ONE DAY, DURING a three-week workshop, Knight stood beneath the scaffolding of a two-level structure in Project Cube, designed by Paul Keogan, and led his three musicians through a percussive exercise performed with crumpled paper bags and tattered sheets of bubble wrap. The rustling and shredding formed a complicated but comprehensible beat pattern, to which the percussionist Robbie Harris added counter-rhythms from an unconventional array of drums.

Picking around an open performance space, trimmed with banks of electronic equipment, musical instruments and what seemed to be an enormous skewer of human hair (it was later identified as an enormous skewer of human hair), the quartet ultimately took their drum sticks to the steel girders of the set itself. The group maintained eye contact throughout, but barely spoke. At times their hollow clacks and shrill echoes sounded compelling; at other times horrid. "Well, that was good," concluded Knight. "Whether or not it's useful . . ."

As the project's composer and director, Knight's methods are painstaking, laborious and often frustrating. The improvisations of the day's rehearsal, for instance, ended in a tense stand-off when one musician was unable to comprehend Knight's intentions - "I know that you're asking for something that doesn't develop" - while Knight was apparently unable to explain them. The chaos would make sense, he said opaquely, "when we have the actual slats in."

"I don't want to hide the fact that they are talented musicians," Knight later explains. "The natural thing is to want something to develop rhythmically; tone-wise." But Knight, who records hours of improvisation on video and mini-disk before meticulously sorting through it to decide what works, seemed to be deliberately courting chaos.

"It's a thunderous escape; the door finally opening," Knight had explained to Maki Watanabe, the Butoh dancer whose movements serve to express the feral body. "I have a picture of a door opening - a huge door - and the outside world crashes in."

The following week, when giant wooden slats (recycled from one of the Abbey Theatre's discarded sets) had finally been affixed to the scaffold to form an enormous fence around the performance space, members of the outside world came in for a visit. Peeking through cracks between the boards, they caught glimpses of a figure shifting in the gloom, while atmospheric shivers of sound enveloped her.

Later the audience moved up to the scaffolding's second level, a walkway directly above the musicians, from where they could look down on the stage. Watanabe's Butoh performance - "moving at the speed of a corpse," as Knight described it - was alternately serene and frenzied, and, like so much of slat, not easy to interpret. "She's able to transform from a little girl to a wolf, to a wolf screaming about a little girl, or vice versa," Knight had offered.

In a post-show discussion with a number of New York theatre students, Knight seemed more relaxed when explaining the process - the story of Anna, his idea for the set structure, the final tumult of the outside coming in - while Paul Keogan described the concept of the slats; to conceal as much as reveal. They asked the audience for feedback. The students seemed appreciative, vocal but a little bewildered.

"I liked the music under my feet," said one.

"I felt powerless," offered another, "it was difficult for me to not do something."

"It's interesting to smell the wood," opined one more.

Knight smiled.

"Because it's very abstract, it does take the audience a bit of work to understand what's going on," he admitted.

"Each person in the audience will interpret it in a different way. We're using this couple of weeks not to find a narrative, but to find the parameters within which we work."

AS TIGHT AS those parameters are, they seem generous in comparison to one of Knight's earlier performance ideas - that he would build a confined structure and live inside it for a week, "and people come and visit. That was a notion I had. I'd inhabit the space to see what happens."

One begins to worry how deep his empathy with feral children runs. "To find the real heart of what I'm after," he says slowly, "that may be another experiment I need to do."

The idea is announced quietly, seriously and perfectly in keeping with Knight's character: gentle on the outside, but wild at heart.