Our correlation-based intrinsic functional connectivity approaches Saracatinib cost only measure symmetric (undirected) connections between regions with temporally synchronous BOLD fluctuations. These methods cannot

differentiate direct from indirect links or infer causality (direction of information flow). These limitations apply to all current intrinsic functional network analyses in humans because the true graph (determined at the microscopic level by the presence of axonal connections between regions) cannot be determined with existing methods. We attempted to mitigate these concerns by thresholding the graphs at a stringent statistical threshold, leaving only strong edges for calculation of graph metrics, but this approach does not preclude our edges from representing indirect connections within or outside the network. Despite these limitations, the functional network graphs derived here provide relevant data about network organization. Understanding the cellular and molecular basis for network-based disease spread represents an important priority for neurodegenerative disease research. Human intrinsic connectivity data cannot directly inform cellular pathogenesis models, just as simple laboratory models include assumptions regarding

their relevance to human disease. This study sought to bridge these research streams by translating mechanistic network-based neurodegeneration models into simple but rational predictions PLX3397 in vivo regarding the relationships secondly between network connectivity and vulnerability. Complementary studies using structural connectivity data could further explore connectivity-vulnerability interactions. The present findings suggest that, overall, a transneuronal spread model best accounts for the

network-based vulnerability observed in previous human neuropathological and imaging studies. Several mechanisms of transneuronal spread have been proposed, including axonal transport of undetected viruses or toxins (Hawkes et al., 2007 and Saper et al., 1987). Providing a more parsimonious account, growing evidence suggests that prion-like mechanisms may promote the spread of toxic, misfolded, nonprion protein species between interconnected neurons (Baker et al., 1993, Baker et al., 1994, Brundin et al., 2010, Clavaguera et al., 2009, Frost and Diamond, 2010, Frost et al., 2009, Hansen et al., 2011, Jucker and Walker, 2011, Lee et al., 2010, Li et al., 2008, Ridley et al., 2006 and Walker et al., 2006). This notion, that many or all noninfectious neurodegenerative diseases may propagate along networked axons via templated conformational change, has been put forth since the introduction of the prion concept (Prusiner, 1984 and a).