 His participation in the Courses for New Music in Cologne in 1963 and 1964 under Karlheinz Stockhausen and Henri Pousseur made a strong impact on his style, which changed to include clusters, quarter tones, electronics, and unconventional instrumental combinations (; ).His best-known work is "Twelve Glass Windows of Chagall in Jerusalem", for voice and instruments (1966).In addition, he composed "Thistles", for horn, percussion, piano, and cello (1967), "Pastels" for two pianos, the piano suite "Seven Little Insects" (1956), and "The Grey Colours of Käthe Kollwitz" for mezzo-soprano, chamber orchestra, and tape.