University of Grenoble, TIM 3-Math?matiques Appliqu?es F-38402 St. Martin d'H?res, France University of Heidelberg, Institut f?r Angewandte Mathematik D-6900 Heidelberg, Federal Republic of Germany Zentralinstitut f?r Seelische Gesundheit D-6800 Mannheim 1, Federal Republic of Germany