1. C. S. Carter, T. S. Braver, D. M. Barch, M. M. Botvinick, Western Psychiatric Institute and Clinic, School of Medicine, University of Pittsburgh, 3811 O'Hara Street, Pittsburgh, PA 15213, USA.
2. D. Noll, Magnetic Resonance Research Center, Presbyterian University Hospital, B-804, University of Pittsburgh Health System, 200 Lothrop Street, Pittsburgh, PA 15213, USA.
3. J. D. Cohen, Western Psychiatric Institute and Clinic, School of Medicine, University of Pittsburgh, 3811 O'Hara Street, Pittsburgh, PA 15213, USA, and Department of Psychology, Carnegie Mellon University, 5000 Forbes Avenue, Pittsburgh, PA 15213, USA.