This volume is the outcome of a Symposium held in Lyon, France. The meeting was organized under the auspices of the Institut National de la Sante et de la Recherche Medicale (INSERM, Paris). We are grateful to the Universite Claude-Bernard which allowed us to use the house of the Brothers Lumiere for the site of the meeting. We would also like to acknowledge the generosity of the Fondation Merieux (Lyon) which pro- vided us with a reception at the house where Claude Bernard was born. In addition to the authors ofthis volume we wish to thank the following individuals for their contributions to the success ofthe Symposium: Chris- tine Baleydier, Simon Faugier-Grimaud, Francoise Girardet, Jacqueline Jeannerod, Henry Kennedy, Michele Magnin, Claude Prablanc, Kath~ erine Page, Lawrence Stark and Francois Vital-Durand. Support from the Office of Naval Research (Contract # N00014-80-K- 0243), the National Eye Institute (Grant # 1 P30-EY02621), the Institut National de la Sante et de la Recherche Medicale (Paris) and Sherin Stahl, a participant in the Undergraduate Research Opportunities Program of the Massachusetts Institute of Technology made this volume possible.