1. Department of Neuropathology, University of Dusseldorf, 4000 Düsseldorf 1, West Germany.
2. Department of Neurosurgery, University of Dusseldorf, 4000 Düsseldorf 1, West Germany.
3. Messerschmitt-Bölkow-Blohm Medizintechnik, 8000 Munich 80, West Germany.