This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This work was supported by grants from the following entities: Centre for Networked Biomedical Research into Neurodegenerative Diseases (CIBERNED, CB06/05/0066 to JDF), the Spanish Ministry of Economy and Competitiveness (grants SAF2009-09394 to JDF; Juan de la Cierva postdoctoral fellowship to RA; TIN2010-20900-C04-04 and the Cajal Blue Brain Project, Spanish partner of the Blue Brain Project initiative from EPFL, to JDF, PL and CB. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing interests: The authors have declared that no competing interests exist.