1. Computer Science & Neuroscience, University of Southern California, 3641 Watt Way, Los Angeles, CA 90089–2520, USA
2. ATR Human Information Sciences, 2–2 Hikaridai, Seika–cho, Soraku–gun, Kyoto 619–0218, Japan
3. School of Computer and Communication Sciences, Lausanne, CH 1015 Lausanne, Switzerland
4. School of Engineering, Swiss Federal Institute of Technology, Lausanne, CH 1015 Lausanne, Switzerland