1. Department of Brain and Cognitive Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA.
2. Laboratory for Neural Circuits, Bio-Mimetic Control Research Center, Institute of Physical and Chemical Research (RIKEN), 3-8-31 Rokuban, Atsuta, Nagoya, Aichi 456, Japan.
3. Osaka University, Faculty of Health and Sport Sciences, Machikaneyama, Osaka 560, Japan.