According to Mauricio Kagel‚ ‘The point of departure for Heterophonie was to write an orchestral work for solo instruments in which the notation would always be the same‚ but the realisation in performance would always be new.’ Qualities of positive and negative heterophony – the simultaneous presentation of musical ideas respectively through artistic intention and performer ‘unintention’ – are pervasive. A haze of tuning leads into the first of five parts‚ alternately delicate and ominous. Many of what have become avant­ garde clichés – from detached notes to textural clusters – are exhibited in an almost too subtle parody. The latter parts do not offer a linear or cumulative follow­through‚ rather an intensifying commentary on the material previously heard. There are memorable touches – the glowering organ writing of part two‚ or the finely chiselled textures of part four. Depending on how you respond‚ it is either 34 minutes of frustrating inaction or of constant fascination. As radical orchestral writing of the period‚ however‚ it is less gripping than Stockhausen’s Gruppen‚ less provocative than Berio’s Alleluia II and less alluring than Boulez’s Figures­Doubles­Prismes. An absorbing work even so‚ worth reviving in a performance as lucid and controlled as Gielen’s. Improvisation ajoutée dates from 1961‚ making it contemporary with Ligeti’s Volumina – which it equals in sonic invention‚ but not in sheer power and inevitability of forward motion. To this end‚ the vocal contributions‚ intent on debunking the organist’s every move‚ now seem a hindrance rather than an enhancement. A period piece‚ even in as lively and committed an account as Zacher’s. Those already interested in Kagel will need to have this disc. Newcomers to this fascinating composer should try one of his more recent concert works – the Third String Quartet (Montaigne) or the Three Etudes (Col Legno) – where the coming together of idea and execution is more completely achieved.