We are grateful to Sahotra Sarkar, Josef Hofbauer, Marc Feldman, and Marion Lamb for their constructive comments on this manuscript. We wish to thank the interdisciplinary program for fostering excellence at the Tel-Aviv university for providing a frame for this collaboration. This work was supported by a grant from the Israeli Academy for Science and Humanities, and by a grant from the Tel-Aviv University, to E. J. , and NIH grant GM 28016.