1. Laboratory of Neurophysiology, NIMH, P.O. Box 608, Poolesville, Maryland 20837
2. Vision et Motricité, INSERM U94, 16 Ave. Doyen Lépine, Bron, 69500 France
3. Division of Neurosurgery, Duke University Medical Center, Box 3807, Durham, North Carolina 27710
4. Instituto di Fisiologia umana, Università degli Studi di Roma “la Sapienze,” P. le A. Moro 5, Rome, 1-00185, Italy