Trisha Brown Dance Company, Sadler's Wells, London

Question: when is a dance more like a game of numbers? Answer: when it's by Trisha Brown, choreography's answer to Sudoku. The difficulty comes when the challenge of working out the rules of her game overrides the simple pleasure of watching. The three works presented by the veteran American at Sadler's Wells - created in 1979, 1989 and just this year - might easily have been arranged under the headings elementary, intermediate and advanced. And while there is great satisfaction in cracking the spatial logic that underpins every moment, once you hit the brick wall of incomprehension the result can be a headache.

The main obstacle to pleasure in Glacial Decoy is the startling absence of music. It's also the earliest piece on the programme and the first Brown created for a theatre. Before that she'd sent her dancers across Manhattan rooftops or walking up walls - crazy stuff, but effective in questioning what qualifies an activity as dance. Faced with presenting movement within the aperture of a proscenium, Brown slyly persuades us to imagine what we're seeing as a tiny segment of something bigger, a wide, perhaps infinite chorus line extending to left and right.

In fact there are only five decoys involved, but most of the time we see only two or three of them, with occasional glimpses of elbows or knees to remind us the others are there. Clad in gauzy white nighties and disporting themselves in floppy jumps and skips, these women seem cut from Romantic ballet, yet their precise geometric tracking in forward and reverse is closer to maths or mechanics. The puzzle-element is compounded by Robert Rauschenberg's set, four screens showing changing photo images. There may be a correlation between the frequency and repetitions of pictures and dance, but you can tie your brain in knots trying to spot it.

Even the title of Brown's new work is a challenge. How Long Does the Subject Linger on the Edge of the Volume... employs the latest advances in motion-capture technology to weave movement, music and visual elements into one integrated design. The idea is that digitally-generated projections interact with the living bodies, whose movements in turn trigger the electronic music. In practice however, the tantalising images dreamed up by Shelley Eshkar, Marc Downie and Paul Kaiser grab all the attention. Ghostly outlines of pyramids and cubes bowl across deep black space like comets. Cosmic 3-D windows fizzle into being and dissolve, and giant humanoid phantoms materialise from living forms. Neither the burblings of Curtis Bahn's score nor the fidgety mesh of Brown's choreography is a match for this grand otherworldliness.

The elements have a more equal chance in 1989's Astral Convertible (design again by Rauschenberg), a sleek vehicle for moving parts that shows Brown's cryptic talent at its best. What look like chrome shelf units from Ikea turn out to be sourced from car innards decked with headlamps, the sole source of light. Nine dancers, in skin-hugging platinum, slip in and out of meticulous formations stepping over or under any obstacle in their path. The patterns converge in hair-raising bottlenecks where bodies miss each other by a hair's breadth. It's that kind of tension that keeps my brain alert. Number crunching is for nerds.

jenny.gilbert@independent.co.uk