1. Aging Research Center, Karolinska Institute, 171 77 Stockholm, Sweden.
2. Departments of Integrative Medical Biology and Radiation Sciences, Umeå Center for Functional Brain Imaging (UFBI), 901 87 Umeå, Sweden.
3. Department of Psychology and Logopedics, Åbo Akademi University, FI-20500 Turku, Finland.
4. Turku PET Center, University of Turku, FI-20500 Turku, Finland.
5. Department of Psychology, University of Umeå, 901 87 Umeå, Sweden.