Split Sides, Barbican, London

With the American choreographer Merce Cunningham, there is always something else going on.

With the American choreographer Merce Cunningham, there is always something else going on. Dances overlap. Music, designs and choreography are made independently and brought together for the performance. It sounds impossible, but the dances have extraordinary richness, filling the stage and the mind.

Cunningham, now 85, has been working with chance since the Fifties, using dice or tossed coins to decide aspects of a dance. With Split Sides, made last year, some of the decisions happen on the night. Everything is doubled. The two scores were commissioned from rock bands Radiohead and Sigur Rós; designs by the Turner-prize nominee Catherine Yass and a 19-year-old student, Robert Heishman; two sets of costumes by James Hall; even two lighting plots, by James F Ingalls. Cunningham puts all these doubles into one dance and rolls dice on stage to decide which comes first. There are 32 possible combinations.

The Cunningham company, co-presented by BITE, made a triumphant opening for this year's Dance Umbrella. On the first night, the die was rolled by Siobhan Davies, Richard Alston, Peter Gill and Graham Sheffield. We had Yass's green-and-blue blurs followed by Heishman's monochrome backdrop, loose coloured costumes then silver-white body tights, Radiohead's sampled speech and hums before Sigur Rós's chimes and rattles.

The steps are full of changes, too. Cunningham's style combines sculptural clarity and drastic contrast. Dancers dip sideways, plunge forwards, sweep through radical shifts of rhythm.This is virtuoso dancing.In a marvellous solo, Jonah Bokaer holds rigid poses, breaking them with sudden undulations.

The mood is as mercurial as the steps. In one duet, Holley Farmer and Daniel Squire move with marionette quickness - their fast feet and tilted bodies have a staccato force. Then the dance slows, and a whole emotional landscape changes with it.

Why does that make such a difference? It isn't a marked change of pace, yet these longer phrases glow with communicative warmth. Farmer's flicked feet seem conversational; she falls back against Squire, or stretches over his back. For all their physical closeness, they barely look at each other, hardly need to. At last, Farmer curls forwards, kicking her legs up behind as he lifts her. She's like a woman wading into surf, carried by the waves.

Other dances are changed by their surroundings. A quintet for four women, bending and curving, and one man, who stands still in the middle. Then he moves, too, with feet turned out, a quizzical penguin

The programme opens with How to Pass, Kick, Fall and Run. Here the dancers are witty athletes, arranging themselves in duet poses before falling gracefully flat. It's accompanied by John Cage stories, sharp and funny, read by Cunningham and company archivist David Vaughan. Sometimes the stories overlap, just like the dances.

Barbican Theatre, London, EC2, until tomorrow (0845 120 7518)