1. School of Computer Science, Carnegie Mellon University, Pittsburgh, PA 15213.
2. Department of Psychiatry, School of Medicine, University of Pittsburgh, Pittsburgh, PA 15213.
3. Digital Equipment Corporation, Paris Research Laboratory, 92563 Rueil-Malmaison Cedex, France.
4. Departnent of Psychology, Carnegie Mellon University, Pittsburgh, PA 15213, and Department of Psychiatry and Behavioral Sciences, School of Medicine, Stanford University, Stanford, CA 94305.