. Rich-club organization of the human connectome. J Neurosci. 2011 Nov 2;31(44):15775-86. PubMed.

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  1. Just about everyone knows a person who seems to know an unusually large number of people, has the latest information on how they are doing, and can quickly put them in touch with a potential friend or colleague. In the study of social network theory, these people are known as "hubs" or "connectors." Similarly, the human brain appears to be organized so that a small number of key brain regions have many connections to other regions of the brain and likely act as hubs to facilitate the transfer of information. This pattern of connectivity can be measured by examining the functional connections among brain regions (measuring how their activity is spontaneously correlated; see Buckner et al., 2009), or by examining the structural connections via which neural signals are transmitted. The paper by van den Heuvel and Sporns uses a technique to measure the white matter fiber tracts that make up the structural connectivity of the brain.

    They find that a small set of brain regions have both a high degree of interconnectedness with other regions of the brain and a high degree of crosstalk with one another. This they term a so-called "rich club" (regions rich in connections that meet together in a semi-private club). Not surprisingly, these rich-club regions happen to be key structures in the relay of information (e.g., the thalamus has long been known to relay sensory information to cortical regions) or are positioned at junctures of major white matter fiber tracts (e.g., the precuneus is positioned at a juncture of the corpus callosum, which connects the brain's hemispheres, and the superior longitudinal fasciculus, which connects the frontal lobe with the parietal and occipital lobes).

    They model the effects of selectively damaging the interconnections between these rich-club regions and find that such damage is particularly devastating to the shortest-path connections between two regions. In other words, when rich-club regions are severed from one another, information can only flow between regions along much less efficient routes. It is important to point out that this is a hypothetical model only, as such severe trauma to the densest white matter tracts is rare and usually only seen in cases of last-resort surgery. The brain is also a highly adaptive organ, and can partially "rewire" itself in response to milder trauma.

    Of interest to the study of Alzheimer's disease, one of the regions that is a member of the rich-club, the hippocampus, is both central to memory formation and highly susceptible to the neurofibrillary tangles found in AD. AD may therefore disproportionately affect the ability of other regions of the brain to interact with the hippocampus through its rich-club connections. Other work has shown that there is a high degree of overlap in the accumulation of amyloid plaques and areas of the neocortex with a high degree of connectivity (including many of the same areas identified here as the rich-club), so AD may have a second route of impact by destroying the ability of these neocortical regions to interact with one another and with the hippocampus.

    References:

    . Cortical hubs revealed by intrinsic functional connectivity: mapping, assessment of stability, and relation to Alzheimer's disease. J Neurosci. 2009 Feb 11;29(6):1860-73. PubMed.