1 Wellcome Trust-Medical Research Council (MRC) Stem Cell Institute, John van Geest Centre for Brain Repair, and Department of Veterinary Medicine, University of Cambridge, Cambridge CB2 1QR, United Kingdom. Electronic address: rk385@cam.ac.uk.
2 Research Group for Lifespan Changes in Brain and Cognition, Department of Psychology, Univeristy of Oslo, POB 1094 Blindern, 0317 Oslo, Norway; Department of Physical Medicine and Rehabilitation, Unit of Neuropsychology, Oslo University Hospital, Oslo, Norway.