1. Department of Psychology and Neuroscience Program, University of Iowa, Iowa City, IA 52242, USA.
2. Laboratory of Neuropsychology, National Institute of Mental Health, Bethesda, MD 20892, USA.
3. Center for Perceptual Systems, University of Texas, Austin, TX 78712, USA.
4. Laboratory of Cerebral Metabolism, National Institute of Mental Health, Bethesda, MD 20892, USA.