1. M. A. Just, P. A. Carpenter, T. A. Keller, Department of Psychology, Carnegie Mellon University, Pittsburgh, PA 15213, USA.
2. W. F. Eddy, Department of Statistics, Carnegie Mellon University, Pittsburgh, PA 15213, USA.
3. K. R. Thulborn, Department of Radiology, University of Pittsburgh Medical Center, Pittsburgh, PA 15213, USA.