Schoenberg's Pierrot Lunaire is one of the masterpieces of early modernism, but even though it's nearly a century old, it still has the power to shock. And it is one of the trickiest works to pull off: the reciter must master Schoenberg's sprechgesang, a style of delivery that lies somewhere between singing and speaking. Anna Dennis's performance with the Manchester-based new music group Psappha answered some of the problems but left others unresolved.

Schoenberg's scoring is a miracle of instrumental colour. Somehow, using just six players, he conjures the grotesque, nightmarish world of Albert Giraud's poems, with their moon-mad imagery of Pierrot drilling into the skull of Cassander and filling it with Turkish tobacco, of the moon dripping with wine, of his dreams of his own execution.

In one song, Psappha revealed Schoenberg's matchless evocation of the nocturnal work of a washerwoman, with the delicate weaving of a violin and flute duet. In another, a solo flute conjured the feverish symbolism of a sick moon. But even more powerful was the tenebrous combination of bass clarinet, cello and low piano chords in the next number, creating the giant black moths that covered the sun.

Psappha dramatised Schoenberg's world of extreme emotion, turning the instrumental parts into the sounds of Pierrot's nightmares. It was as if they, not Anna Dennis, were playing the part of Pierrot: Dennis's delivery was lyrical and fastidious, but her tendency to turn Schoenberg's sprechgesang into straight song meant that there was something too careful, too singerly about her Pierrot. Instead of inhabiting the role, she performed the piece as a heightened song-cycle, and the terror of Pierrot's dreams was toned down. However, in the final numbers, as Pierrot reflects on his visions, she caught the moonstruck melancholy of Schoenberg's vocal line, its haunting air "from fairytale times".