Morton Feldman's For Philip Guston, written in 1984, is one of the mythical masterworks of his late period: a slow, quiet trio for flute, piano and percussion that lasts four and a half hours. Its latest interpretation has more claims to authenticity than any other performance it has ever received in this country. In the middle of the Royal Academy's The Art of Philip Guston exhibition, pianist Nicolas Hodges, flautist Mario Caroli, and percussion player Richard Benjafield played, surrounded by the art that inspired the work.

Feldman describes the piece as a reflection of the experience of seeing Guston's works for the first time, and the warped repetitions of the music made a musical analogy for the repeated brushstrokes and intense concentration of Guston's early paintings. Allowing the audience to walk around the exhibition during the performance made it possible to connect the music with Guston's paintings.

However, the delicate austerity of the music seemed to inhabit a different creative world from the visceral, politicised violence of Guston's later works - a move from abstraction to representation that alienated his fellow painters and Feldman himself. Ironically, the performance took place in the central room of the exhibition, full of the bulging eyes, diseased Nixons, and mysterious cigars of Guston's huge, cartoon-like canvasses.

But as time passed, something strange happened to the performance. The sounds seemed to assume the physical form of objects, as if the players were creating a gigantic, musical mobile. Even stranger, the work's structure seemed miniature rather than massive, becoming a series of moments that animated the present instead of creating a huge, imposing form. Hearing motives return, such as a tolling vibraphone note, or a three-note flute phrase, after a temporal distance of two or three hours was a bizarre experience, making the music at once familiar and uncanny. The players' concentration was absolute, a virtuosic performance in time as well as technique.