Acknowledgments

Grateful thanks are due to the publishers and editors of the following journals for their generosity in giving permission for reproduction of figures: Annual Review of Neuroscience (Annual Reviews), Journal of Neurophysiology (American Physiological Society), Journal of Neuroscience (Society for Neuroscience), Journal of Physiology London (The Physiological Society, John Wiley & Sons), Nature Reviews Neuroscience (Nature Publishing Group), Proceedings of National Academy of Sciences USA (National Academy of Sciences USA), and Progress in Brain Research (Elsevier). Grateful thanks also goes to the Instituto de Neurobiologóa “Ramón y Cajal,” Madrid, Spain.